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  <DIV style="font-family: 'Times New Roman',Times,serif">
  
  
  
  <DIV align="center" style="font-size: 10pt; margin-top: 18pt">As
  filed with the Securities and Exchange Commission on October 7, 2005
  </DIV>
  
  
  <DIV align="right" style="font-size: 10pt; margin-top: 12pt">Registration No.&nbsp;333-_____
  </DIV>
  
  
  <DIV style="width: 100%; border-bottom: 2pt solid black; font-size: 1pt">&nbsp;</DIV>
  <DIV style="width: 100%; border-bottom: 1pt solid black; font-size: 1pt">&nbsp;</DIV>
  
  
  
  
  <DIV align="center" style="font-size: 14pt; margin-top: 12pt"><B>SECURITIES AND EXCHANGE COMMISSION</B>
  </DIV>
  
  <DIV align="center" style="font-size: 12pt"><B>Washington, D.C. 20549</B>
  </DIV>
  
  <DIV align="center" style="font-size: 18pt; margin-top: 12pt"><B>FORM S-8</B>
  </DIV>
  
  <DIV align="center" style="font-size: 10pt; margin-top: 12pt"><B>REGISTRATION STATEMENT UNDER<BR>
  THE SECURITIES ACT OF 1933</B></DIV>
  
  <DIV align="center" style="font-size: 24pt; margin-top: 12pt"><B>Idenix Pharmaceuticals, Inc.</B>
  </DIV>
  
  <DIV align="center" style="font-size: 10pt">(Exact Name of Registrant as Specified in Its Charter)</DIV>
  
  
  <DIV align="center">
  <TABLE style="font-size: 10pt" cellspacing="0" border="0" cellpadding="0" width="100%">
  <!-- Begin Table Head -->
  <TR valign="bottom">
      <TD width="47%">&nbsp;</TD>
      <TD width="5%">&nbsp;</TD>
      <TD width="47%">&nbsp;</TD>
  </TR>
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  <!-- Begin Table Body -->
  <TR valign="bottom">
      <TD align="center" valign="top"><B>Delaware</B>
  </TD>
      <TD>&nbsp;</TD>
      <TD align="center" valign="top"><B>45-0478605</B></TD>
  </TR>
  <TR valign="bottom">
      <TD align="center" valign="top">(State or Other Jurisdiction of Incorporation
  </TD>
      <TD>&nbsp;</TD>
      <TD align="center" valign="top">(I.R.S. Employer</TD>
  </TR>
  <TR valign="bottom">
      <TD align="center" valign="top">or Organization)
  </TD>
      <TD>&nbsp;</TD>
      <TD align="center" valign="top">Identification No.)</TD>
  </TR>
  
  
  <TR><TD align="left" valign="top">&nbsp;</TD></TR>
  
  <TR valign="bottom">
      <TD align="center" valign="top">60 Hampshire Street</TD>
      <TD>&nbsp;</TD>
      <TD align="center" valign="top">02139</TD>
  </TR>
  <TR valign="bottom">
      <TD align="center" valign="top">Cambridge, MA
  </TD>
      <TD>&nbsp;</TD>
      <TD align="center" valign="top">(Zip Code)</TD>
  </TR>
  <TR valign="bottom">
      <TD align="center" valign="top">(Address of Principal Executive Offices)
  </TD>
      <TD>&nbsp;</TD>
      <TD align="center" valign="top"></TD>
  </TR>
  <!-- End Table Body -->
  </TABLE>
  </DIV>
  
  <DIV align="center" style="font-size: 10pt; margin-top: 12pt"><B>Idenix Pharmaceuticals, Inc.<BR>
  2005 Stock Incentive Plan</B><BR>
  (Full Title of the Plan)</DIV>
  
  <DIV align="center" style="font-size: 10pt; margin-top: 12pt"><B>Jean-Pierre Sommadossi<BR>
  President and Chief Executive Officer<BR>
  Idenix Pharmaceuticals, Inc.<BR>
  60 Hampshire Street<BR>
  Cambridge, MA 02139</B><BR>
  (Name and Address of Agent For Service)<BR>
  <B>617-995-9800</B><BR>
  (Telephone Number, Including Area Code, of Agent For Service)</DIV>
  
  <DIV align="center" style="font-size: 10pt; margin-top: 12pt"><B>CALCULATION OF REGISTRATION FEE</B></DIV>
  
  <DIV align="center">
  <TABLE style="font-size: 10pt" cellspacing="0" border="0" cellpadding="0" width="100%">
  <!-- Begin Table Head -->
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      <TD width="1%">&nbsp;</TD>
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      <TD width="2%">&nbsp;</TD>
      <TD width="7%">&nbsp;</TD>
  
      <TD width="1%">&nbsp;</TD>
  </TR>
  
  <TR style="font-size: 1px" valign="bottom">
      <TD nowrap align="left" colspan="15" style="border-bottom: 1px solid #000000">&nbsp;</TD>
  </TR>
  
  <TR style="font-size: 8pt" valign="bottom">
  <TD width="1%">&nbsp;</TD>
      <TD nowrap align="left"></TD>
      <TD style="border-right: 1px solid #000000">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD style="border-right: 1px solid #000000">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD nowrap align="center">Proposed Maximum</TD>
      <TD style="border-right: 1px solid #000000">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD nowrap align="center">Proposed Maximum</TD>
      <TD style="border-right: 1px solid #000000">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD nowrap align="center">&nbsp;</TD>
      <TD width="1%">&nbsp;</TD>
  </TR>
  
  <TR style="font-size: 8pt" valign="bottom">
  <TD width="1%">&nbsp;</TD>
      <TD nowrap align="left"></TD>
      <TD style="border-right: 1px solid #000000">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD nowrap align="center">Amount to be</TD>
      <TD style="border-right: 1px solid #000000">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD nowrap align="center">Offering Price Per</TD>
      <TD style="border-right: 1px solid #000000">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD nowrap align="center">Aggregate Offering</TD>
      <TD style="border-right: 1px solid #000000">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD nowrap align="center">Amount of</TD>
      <TD width="1%">&nbsp;</TD>
  </TR>
  
  <TR style="font-size: 8pt" valign="bottom">
   <TD width="1%">&nbsp;</TD>
      <TD nowrap align="left">Title of Securities to be Registered</TD>
      <TD style="border-right: 1px solid #000000">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD nowrap align="center">Registered(1)</TD>
      <TD style="border-right: 1px solid #000000">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD nowrap align="center">Share</TD>
      <TD style="border-right: 1px solid #000000">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD nowrap align="center">Price</TD>
      <TD style="border-right: 1px solid #000000">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD nowrap align="center">Registration Fee</TD>
      <TD width="1%">&nbsp;</TD>
  </TR>
  
  <!-- End Table Head -->
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      <TD width="1%" style="border-top: 1px solid #000000">&nbsp;</TD>
                      <TD valign="top" style="border-top: 1px solid #000000"><DIV style="margin-left:0px; text-indent:-0px">Common Stock,
  $0.001 par value
  per share
  </DIV></TD>
      <TD style="border-right: 1px solid #000000; border-top: 1px solid #000000">&nbsp;</TD>
      <TD style="border-top: 1px solid #000000">&nbsp;</TD>
  
  <TD align="center" valign="top" style="border-top: 1px solid #000000" nowrap>2,300,000 shares (2)
  </TD>
      <TD style="border-right: 1px solid #000000; border-top: 1px solid #000000">&nbsp;</TD>
      <TD style="border-top: 1px solid #000000">&nbsp;</TD>
  
  <TD align="center" valign="top" style="border-top: 1px solid #000000">$26.29 (3)
  </TD>
      <TD style="border-right: 1px solid #000000; border-top: 1px solid #000000">&nbsp;</TD>
      <TD style="border-top: 1px solid #000000">&nbsp;</TD>
  
  <TD align="center" valign="top" style="border-top: 1px solid #000000">$60,467,000 (3)
  </TD>
      <TD style="border-right: 1px solid #000000; border-top: 1px solid #000000">&nbsp;</TD>
      <TD style="border-top: 1px solid #000000">&nbsp;</TD>
  
  <TD align="center" valign="top" style="border-top: 1px solid #000000">$7,122</TD>
      <TD width="1%" style="border-top: 1px solid #000000">&nbsp;</TD>
  </TR>
  <TR style="font-size: 1px" valign="bottom">
      <TD nowrap align="left" colspan="15" style="border-top: 1px solid #000000">&nbsp;</TD>
  </TR>
  
  <!-- End Table Body -->
  </TABLE>
  </DIV>
  
  <DIV style="margin-top: 6pt">
  <TABLE width="100%" border="0" cellpadding="0" cellspacing="0" style="font-size: 10pt">
  
  <TR valign="top" style="font-size: 10pt; color: #000000; background: transparent">
      <TD width="5%" nowrap align="left">(1)</TD>
      <TD width="1%">&nbsp;</TD>
      <TD>In accordance with Rule&nbsp;416 under the Securities Act of 1933, as amended, this registration
  statement shall be deemed to cover any additional securities that may from time to time be
  offered or issued to prevent dilution resulting from stock splits, stock dividends or similar
  transactions.</TD>
  </TR>
  
  <TR>
      <TD style="font-size: 6pt">&nbsp;</TD>
  </TR><TR valign="top" style="font-size: 10pt; color: #000000; background: transparent">
      <TD width="5%" nowrap align="left">(2)</TD>
      <TD width="1%">&nbsp;</TD>
      <TD>Includes: (i)&nbsp;8,000 shares of common stock originally reserved for issuance under the
  Registrant&#146;s 2004 Stock Incentive Plan (the &#147;2004 Plan&#148;), (ii)&nbsp;shares of common stock (not to
  exceed 792,000 additional shares) that are subject to awards granted under the 2004 Plan that
  expire, terminate or are otherwise cancelled without issuance of common stock; and (iii)
  1,500,000 shares of common stock originally reserved for issuance pursuant to the Registrant&#146;s
  2005 Stock Incentive Plan.</TD>
  </TR>
  
  <TR>
      <TD style="font-size: 6pt">&nbsp;</TD>
  </TR><TR valign="top" style="font-size: 10pt; color: #000000; background: transparent">
      <TD width="5%" nowrap align="left">(3)</TD>
      <TD width="1%">&nbsp;</TD>
      <TD>Estimated solely for the purpose of calculating the registration fee pursuant to Rules 457(c)
  and 457(h) of the Securities Act of 1933, as amended, and based upon the average of the high
  and low prices of the Registrant&#146;s Common Stock as reported on The NASDAQ National Market on
  October&nbsp;5, 2005.</TD>
  </TR>
  
  </TABLE>
  </DIV>
  
  <DIV style="width: 100%; border-bottom: 1pt solid black; margin-top: 10pt; font-size: 1pt">&nbsp;</DIV>
  <DIV style="width: 100%; border-bottom: 2pt solid black; font-size: 1pt">&nbsp;</DIV>
  
  
  
  
  
  <P align="center" style="font-size: 10pt">&nbsp;
  </DIV>
  
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  <DIV style="font-family: 'Times New Roman',Times,serif">
  
  <!-- TOC -->
  <A name="toc"><DIV align="CENTER" style="page-break-before:always"><U><B>TABLE OF CONTENTS</B></U></DIV></A>
  
  <P><CENTER>
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  	<TD width="3%"></TD>
  	<TD width="76%"></TD>
  </TR>
  <TR><TD colspan="9"><A HREF="#000">PART I</A></TD></TR>
  <TR><TD colspan="9"><A HREF="#001">PART II</A></TD></TR>
  <TR><TD colspan="9"><A HREF="#002">SIGNATURES</A></TD></TR>
  <TR><TD colspan="9"><A HREF="b57097ipexv5w1.htm">EX-5.1 Opinion of Wilmer Cutler Pickering Hale and Dorr LLP</A></TD></TR>
  <TR><TD colspan="9"><A HREF="b57097ipexv23w2.htm">EX-23.2 Consent of PWC LLP</A></TD></TR>
  </TABLE>
  </CENTER>
  <!-- /TOC -->
  <P><HR noshade><P>
  <H5 align="left" style="page-break-before:always"><A HREF="#toc">Table of Contents</A></H5><P>
  
  <!-- link1 "PART I" -->
  <DIV align="left"><A NAME="000"></A></DIV>
  <DIV align="center" style="font-size: 10pt; margin-top: 18pt"><B>PART I<BR>
  INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS</B>
  </DIV>
  
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<B>Item&nbsp;1. Plan Information.</B>
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;The information required by Item&nbsp;1 is included in documents sent or given to participants in
  the plans covered by this registration statement pursuant to Rule&nbsp;428(b)(1) of the Securities Act
  of 1933, as amended (the &#147;Securities Act&#148;).
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<B>Item&nbsp;2. Registrant Information and Employee Plan Annual Information.</B>
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;The written statement required by Item&nbsp;2 is included in documents sent or given to
  participants in the plans covered by this registration statement pursuant to Rule&nbsp;428(b)(1) of the
  Securities Act.
  </DIV>
  
  <!-- link1 "PART II" -->
  <DIV align="left"><A NAME="001"></A></DIV>
  <DIV align="center" style="font-size: 10pt; margin-top: 18pt"><B>PART II<BR>
  INFORMATION REQUIRED IN THE REGISTRATION STATEMENT</B>
  </DIV>
  
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<B>Item&nbsp;3. Incorporation of Documents by Reference.</B>
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;The registrant is subject to the informational and reporting requirements of Sections&nbsp;13(a),
  14, and 15(d) of the Securities Exchange Act of 1934, as amended (the &#147;Exchange Act&#148;), and in
  accordance therewith files reports, proxy statements and other information with the Securities and
  Exchange Commission (the &#147;Commission&#148;). The following documents, which are on file with the
  Commission, are incorporated in this registration statement by reference:
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(a)&nbsp;The registrant&#146;s latest annual report filed pursuant to Section 13(a) or 15(d) of the
  Exchange Act or the latest prospectus filed pursuant to Rule 424(b) under the Securities Act that
  contains audited financial statements for the registrant&#146;s latest fiscal year for which such
  statements have been filed.
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(b)&nbsp;All other reports filed pursuant to Section 13(a) or 15(d) of the Exchange Act since the
  end of the fiscal year covered by the document referred to in (a)&nbsp;above.
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(c)&nbsp;The description of the securities contained in the registrant&#146;s registration statements on
  Form 8-A filed under the Exchange Act, including any amendment or report filed for the purpose of
  updating such description.
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;All documents subsequently filed by the registrant pursuant to Sections&nbsp;13(a), 13(c), 14 and
  15(d) of the Exchange Act, prior to the filing of a post-effective amendment which indicates that
  all securities offered hereby have been sold or which deregisters all securities then remaining
  unsold, shall be deemed to be incorporated by reference in this registration statement and to be
  part hereof from the date of the filing of such documents. Any statement contained in a document
  incorporated or deemed to be incorporated by reference herein shall be deemed to be modified or
  superseded for the purposes of this registration statement to the extent that a statement contained
  herein or in any other subsequently filed document which also is or is deemed to be incorporated by
  reference herein modifies or supersedes such statement. Any statement so modified or superseded
  shall not be deemed, except as so modified or superseded, to constitute a part of this registration
  statement.
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<B>Item&nbsp;4. Description of Securities.</B>
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Not applicable.
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<B>Item&nbsp;5. Interests of Named Experts and Counsel.</B>
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Wilmer Cutler Pickering Hale and Dorr LLP has opined as to the legality of the securities
  being offered by this registration statement. Ambassador Charlene Barshefsky, a partner in the law
  firm of Wilmer Cutler Pickering Hale and Dorr LLP beneficially owns 61,343 shares of the
  registrant&#146;s common stock.
  </DIV>
  
  <P align="center" style="font-size: 10pt">&nbsp;
  </DIV>
  
  <!-- PAGEBREAK -->
  <P><HR noshade><P>
  <H5 align="left" style="page-break-before:always"><A HREF="#toc">Table of Contents</A></H5><P>
  
  <DIV style="font-family: 'Times New Roman',Times,serif">
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<B>Item&nbsp;6. Indemnification of Directors and Officers.</B>
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Section&nbsp;145 of the General Corporation Law of the State of Delaware permits a corporation,
  under specified circumstances, to indemnify its directors, officers, employees or agents against
  expenses (including attorney&#146;s fees), judgments, fines and amounts paid in settlements actually and
  reasonably incurred by them in connection with any action, suit or proceeding brought by third
  parties by reason of the fact that they were or are directors, officers, employees or agents of the
  corporation, if such directors, officers, employees or agents acted in good faith and in a manner
  they reasonably believed to be in or not opposed to the best interests of the corporation and, with
  respect to any criminal action or proceeding, had no reason to believe their conduct was unlawful.
  In a derivative action, i.e., one by or in the right of the corporation, indemnification may be
  made only for expenses actually or reasonably incurred by directors, officers, employees or agents
  in connection with the defense or settlement of an action or suit, and only with respect to a
  matter as to which they shall have acted in good faith and in a manner they reasonably believed to
  be in or not opposed to the best interests of the corporation, except that no indemnification shall
  be made if such person shall have been adjudged liable to the corporation, unless and only to the
  extent that the court in which the action or suit was brought shall determine upon application that
  the defendant directors, officers, employees or agents are fairly and reasonably entitled to
  indemnify for such expenses despite such adjudication of liability.
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;The registrant&#146;s restated certificate of incorporation, as amended, states that the
  registrant&#146;s directors will not have personal liability for monetary damages for any breach of
  fiduciary duty as a director, except to the extent that the General Corporation Law of the State of
  Delaware prohibits the elimination or limitation of liability of directors for breaches of
  fiduciary duty.
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;The registrant also carries liability insurance covering each of its directors and officers.
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<B>Item&nbsp;7. Exemption from Registration Claimed.</B>
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Not applicable.
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<B>Item&nbsp;8. Exhibits.</B>
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;The Exhibit&nbsp;Index immediately preceding the exhibits is incorporated herein by reference.
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<B>Item&nbsp;9. Undertakings.</B>
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;1.&nbsp;<U><I>Item&nbsp;</I><I>512(a)</I><I> of Regulation&nbsp;S-K</I></U>. The undersigned registrant hereby undertakes:
  </DIV>
  
  <DIV style="margin-top: 6pt">
  <TABLE width="100%" border="0" cellpadding="0" cellspacing="0" style="font-size: 10pt">
  
  <TR valign="top" style="font-size: 10pt; color: #000000; background: transparent">
      <TD width="6%" style="background: transparent">&nbsp;</TD>
      <TD width="3%" nowrap align="left">(1)</TD>
      <TD width="1%">&nbsp;</TD>
      <TD>To file, during any period in which offers or sales are being made, a
  post-effective amendment to this registration statement:</TD>
  </TR>
  
  </TABLE>
  </DIV>
  
  <DIV style="margin-top: 6pt">
  <TABLE width="100%" border="0" cellpadding="0" cellspacing="0" style="font-size: 10pt">
  
  <TR valign="top" style="font-size: 10pt; color: #000000; background: transparent">
      <TD width="6%" style="background: transparent">&nbsp;</TD>
      <TD width="3%" nowrap align="left">(i)</TD>
      <TD width="1%">&nbsp;</TD>
      <TD>To include any prospectus required by Section&nbsp;10(a)(3) of the Securities Act;</TD>
  </TR>
  
  <TR>
      <TD style="font-size: 6pt">&nbsp;</TD>
  </TR><TR valign="top" style="font-size: 10pt; color: #000000; background: transparent">
      <TD width="6%" style="background: transparent">&nbsp;</TD>
      <TD width="3%" nowrap align="left">(ii)</TD>
      <TD width="1%">&nbsp;</TD>
      <TD>To reflect in the prospectus any facts or events arising after the effective date
  of the registration statement (or the most recent post-effective amendment thereof) which,
  individually or in the aggregate, represent a fundamental change in the information set
  forth in the registration statement; and</TD>
  </TR>
  
  <TR>
      <TD style="font-size: 6pt">&nbsp;</TD>
  </TR><TR valign="top" style="font-size: 10pt; color: #000000; background: transparent">
      <TD width="6%" style="background: transparent">&nbsp;</TD>
      <TD width="3%" nowrap align="left">(iii)</TD>
      <TD width="1%">&nbsp;</TD>
      <TD>To include any material information with respect to the plan of distribution not
  previously disclosed in the registration statement or any material change to such
  information in the registration statement;</TD>
  </TR>
  
  </TABLE>
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<I>provided, however, </I>that paragraphs (i)&nbsp;and (ii)&nbsp;do not apply if the information required to
  be included in a post-effective amendment by those paragraphs is contained in periodic
  reports filed with or furnished to the Commission by the registrant pursuant to Section&nbsp;13
  or Section 15(d) of the Exchange Act that are incorporated by reference in the registration
  statement.
  </DIV>
  
  <P align="center" style="font-size: 10pt">&nbsp;
  </DIV>
  
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  <H5 align="left" style="page-break-before:always"><A HREF="#toc">Table of Contents</A></H5><P>
  
  <DIV style="font-family: 'Times New Roman',Times,serif">
  
  <DIV style="margin-top: 6pt">
  <TABLE width="100%" border="0" cellpadding="0" cellspacing="0" style="font-size: 10pt">
  
  <TR valign="top" style="font-size: 10pt; color: #000000; background: transparent">
      <TD width="6%" style="background: transparent">&nbsp;</TD>
      <TD width="3%" nowrap align="left">(2)</TD>
      <TD width="1%">&nbsp;</TD>
      <TD>That, for the purpose of determining any liability under the Securities Act, each
  such post-effective amendment shall be deemed to be a new registration statement relating to
  the securities offered therein, and the offering of such securities at that time shall be
  deemed to be the initial <I>bona fide </I>offering thereof.</TD>
  </TR>
  
  <TR>
      <TD style="font-size: 6pt">&nbsp;</TD>
  </TR><TR valign="top" style="font-size: 10pt; color: #000000; background: transparent">
      <TD width="6%" style="background: transparent">&nbsp;</TD>
      <TD width="3%" nowrap align="left">(3)</TD>
      <TD width="1%">&nbsp;</TD>
      <TD>To remove from registration by means of a post-effective amendment any of the
  securities being registered which remain unsold at the termination of the offering.</TD>
  </TR>
  
  </TABLE>
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;2.&nbsp;<U><I>Item&nbsp;</I><I>512(b)</I><I> of Regulation&nbsp;S-K</I></U>. The undersigned registrant hereby undertakes that,
  for purposes of determining any liability under the Securities Act, each filing of the registrant&#146;s
  annual report pursuant to Section 13(a) or Section 15(d) of the Exchange Act that is incorporated
  by reference in the registration statement shall be deemed to be a new registration statement
  relating to the securities offered therein, and the offering of such securities at that time shall
  be deemed to be the initial <I>bona fide </I>offering thereof.
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;3.&nbsp;<U><I>Item&nbsp;</I><I>512(h)</I><I> of Regulation&nbsp;S-K</I></U>. Insofar as indemnification for liabilities arising
  under the Securities Act may be permitted to directors, officers and controlling persons of the
  registrant pursuant to the foregoing provisions, or otherwise, the registrant has been advised that
  in the opinion of the Securities and Exchange Commission such indemnification is against public
  policy as expressed in the Securities Act and is, therefore, unenforceable. In the event that a
  claim for indemnification against such liabilities (other than the payment by the registrant of
  expenses incurred or paid by a director, officer or controlling person of the registrant in the
  successful defense of any action, suit or proceeding) is asserted by such director, officer or
  controlling person in connection with the securities being registered, the registrant will, unless
  in the opinion of its counsel the matter has been settled by controlling precedent, submit to a
  court of appropriate jurisdiction the question whether such indemnification by it is against public
  policy as expressed in the Securities Act and will be governed by the final adjudication of such
  issue.
  </DIV>
  
  <P align="center" style="font-size: 10pt">&nbsp;
  </DIV>
  
  <!-- PAGEBREAK -->
  <P><HR noshade><P>
  <H5 align="left" style="page-break-before:always"><A HREF="#toc">Table of Contents</A></H5><P>
  
  <DIV style="font-family: 'Times New Roman',Times,serif">
  
  <!-- link1 "SIGNATURES" -->
  <DIV align="left"><A NAME="002"></A></DIV>
  <DIV align="center" style="font-size: 10pt; margin-top: 18pt"><B>SIGNATURES</B>
  </DIV>
  
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Pursuant to the requirements of the Securities Act of 1933, the registrant certifies that it
  has reasonable grounds to believe that it meets all of the requirements for filing on Form S-8 and
  has duly caused this registration statement to be signed on its behalf by the undersigned,
  thereunto duly authorized, in the City of Cambridge, Commonwealth of
  Massachusetts, on this 7<sup>th</SUP> day
  of October, 2005.
  </DIV>
  
  <TABLE width="100%" border="0" cellspacing="0" cellpadding="0" style="font-size: 10pt">
  <TR>
      <TD width="48%">&nbsp;</TD>
      <TD width="1%">&nbsp;</TD>
      <TD width="1%">&nbsp;</TD>
      <TD width="35%">&nbsp;</TD>
      <TD width="15%">&nbsp;</TD>
  </TR>
  <TR>
      <TD valign="top" align="left">&nbsp;</TD>
      <TD colspan="3" align="left">IDENIX PHARMACEUTICALS, INC.<BR>
  &nbsp;</TD>
      <TD>&nbsp;</TD>
  </TR>
  
  
  
  <TR>
      <TD align="left">&nbsp;</TD>
      <TD valign="top">By:&nbsp;</TD>
  
  <TD colspan="2" align="left" style="border-bottom:1px solid #000000;">Jean-Pierre Sommadossi</TD>
      <TD>&nbsp;</TD>
  </TR>
  
  
  
  <TR>
      <TD align="left">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD colspan="2" align="left">Jean-Pierre Sommadossi&nbsp;</TD>
      <TD>&nbsp;</TD>
  </TR><TR>
      <TD align="left">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD colspan="2" align="left">President and Chief Executive Officer&nbsp;</TD>
      <TD>&nbsp;</TD>
  </TR>
  <TR>
      <TD colspan="5">&nbsp;</TD>
  </TR>
  </TABLE>
  
  <DIV align="center" style="font-size: 10pt; margin-top: 18pt"><B>POWER OF ATTORNEY AND SIGNATURES</B>
  </DIV>
  
  
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;We, the undersigned officers and directors of Idenix Pharmaceuticals, Inc., hereby severally
  constitute and appoint Jean-Pierre Sommadossi and Andrea J. Corcoran, and each of them singly, our
  true and lawful attorneys with full power to them, and each of them singly, to sign for us and in
  our names in the capacities indicated below, the registration statement on Form S-8 filed herewith
  and any and all subsequent amendments to said registration statement, and generally to do all such
  things in our names and on our behalf in our capacities as officers and directors to enable Idenix
  Pharmaceuticals, Inc. to comply with the provisions of the Securities Act of 1933, as amended, and
  all requirements of the Securities and Exchange Commission, hereby ratifying and confirming our
  signatures as they may be signed by our said attorneys, or any of them, to said registration
  statement and any and all amendments thereto.
  </DIV>
  
  <DIV align="left" style="font-size: 10pt; margin-top: 6pt">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Pursuant to the requirements of the Securities Act of 1933, this registration statement has
  been signed by the following persons in the capacities and on the dates indicated.
  </DIV>
  <DIV align="center">
  <TABLE style="font-size: 10pt" cellspacing="0" border="0" cellpadding="0" width="100%">
  <!-- Begin Table Head -->
  <TR valign="bottom">
      <TD width="32%">&nbsp;</TD>
      <TD width="2%">&nbsp;</TD>
      <TD width="32%">&nbsp;</TD>
      <TD width="2%">&nbsp;</TD>
      <TD width="12%">&nbsp;</TD>
      <TD width="20%">&nbsp;</TD>
  </TR>
  <TR style="font-size: 8pt" valign="bottom">
      <TD nowrap align="left"><U><B>Signature</B></U></TD>
      <TD>&nbsp;</TD>
      <TD nowrap align="center"><U><B>Title</B></U></TD>
      <TD>&nbsp;</TD>
      <TD nowrap align="center"><U><B>Date</B></U></TD>
  </TR>
  
  <!-- End Table Head -->
  <!-- Begin Table Body -->
  
  <TR>
      <TD>&nbsp;</TD>
  </TR>
  
  <TR valign="bottom">
  
  <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">/s/&nbsp;Jean-Pierre Sommadossi</DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">President and Chief Executive</TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top" nowrap>October&nbsp;7, 2005</TD>
  </TR>
  <TR valign="bottom">
      <TD valign="top"><DIV style="border-top: 1px solid #000000"><DIV style="margin-left:0px; text-indent:-0px">Jean-Pierre Sommadossi</DIV></DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">Officer (Principal executive</TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
  </TR>
  <TR valign="bottom">
      <TD valign="top">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">officer)
  </TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
  </TR>
  <TR valign="bottom"><!-- Blank Space -->
      <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">&nbsp;</DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
  </TR>
  <TR valign="bottom">
  
  <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">/s/&nbsp;David A. Arkowitz
  </DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">Chief Financial Officer and</TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">October&nbsp;7, 2005</TD>
  </TR>
  <TR valign="bottom">
      <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px"><DIV style="border-top: 1px solid #000000">David A. Arkowitz</DIV>
  </DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">Treasurer (Principal financial</TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
  </TR>
  <TR valign="bottom">
  
  <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">&nbsp;</DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">and accounting officer)
  </TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
  </TR>
  
  <TR valign="bottom"><!-- Blank Space -->
  
  <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">/s/&nbsp;Charles W. Cramb</DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
  </TR>
  <TR valign="bottom">
      <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px"><DIV style="font-size: 1pt; border-top: 1px solid #000000">&nbsp;</DIV>
  Charles W. Cramb
  </DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">Director
  </TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">October&nbsp;7, 2005</TD>
  </TR>
  
  
  <TR>
      <TD>&nbsp;</TD>
  </TR>
  
  
  <TR valign="bottom"><!-- Blank Space -->
  
  <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">/s/&nbsp;Thomas Ebeling</DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
  </TR>
  <TR valign="bottom">
      <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px"><DIV style="border-top: 1px solid #000000">Thomas Ebeling</DIV>
  </DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">Director
  </TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">October&nbsp;7, 2005</TD>
  </TR>
  <TR valign="bottom">
  
  <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">&nbsp;</DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
  </TR>
  <TR valign="bottom"><!-- Blank Space -->
  
  <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">/s/&nbsp;Wayne T. Hockmeyer</DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
  </TR>
  <TR valign="bottom">
      <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px"><DIV style="border-top: 1px solid #000000">Wayne T. Hockmeyer</DIV>
  </DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">Director
  </TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">October&nbsp;7, 2005</TD>
  </TR>
  <TR valign="bottom">
      <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px"></DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
  </TR>
  <TR valign="bottom"><!-- Blank Space -->
  
  <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">/s/&nbsp;Thomas R. Hodgson</DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
  </TR>
  <TR valign="bottom">
      <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px"><DIV style="border-top: 1px solid #000000">Thomas R. Hodgson</DIV>
  </DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">Director
  </TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">October&nbsp;7, 2005</TD>
  </TR>
  <TR valign="bottom">
  
  <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">&nbsp;</DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
  </TR>
  <!-- End Table Body -->
  </TABLE>
  </DIV>
  
  
  <P align="center" style="font-size: 10pt">&nbsp;
  </DIV>
  
  <!-- PAGEBREAK -->
  <P><HR noshade><P>
  <H5 align="left" style="page-break-before:always"><A HREF="#toc">Table of Contents</A></H5><P>
  
  <DIV style="font-family: 'Times New Roman',Times,serif">
  
  <DIV align="center">
  <TABLE style="font-size: 10pt" cellspacing="0" border="0" cellpadding="0" width="100%">
  <!-- Begin Table Head -->
  <TR valign="bottom">
      <TD width="32%">&nbsp;</TD>
      <TD width="2%">&nbsp;</TD>
      <TD width="32%">&nbsp;</TD>
      <TD width="2%">&nbsp;</TD>
      <TD width="12%">&nbsp;</TD>
      <TD width="20%">&nbsp;</TD>
  </TR>
  
  <!-- End Table Head -->
  <!-- Begin Table Body -->
  
  
  <TR valign="bottom"><!-- Blank Space -->
  
  <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">/s/&nbsp;Robert
  E. Pelzer</DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
  </TR>
  
  
  
  
  
  
  
  
  <TR valign="bottom">
      <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px"><DIV style="border-top: 1px solid #000000">Robert E. Pelzer</DIV>
  </DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">Director
  </TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top" nowrap>October&nbsp;7, 2005</TD>
  </TR>
  <TR valign="bottom">
  
  <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">&nbsp;</DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
  </TR>
  <TR valign="bottom"><!-- Blank Space -->
  
  <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">/s/&nbsp;Denise Pollard-Knight</DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
  </TR>
  <TR valign="bottom">
      <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px"><DIV style="border-top: 1px solid #000000">Denise Pollard-Knight</DIV>
  </DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">Director
  </TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">October&nbsp;7, 2005</TD>
  </TR>
  <TR valign="bottom">
  
  <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">&nbsp;</DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
  </TR>
  
  <TR valign="bottom"><!-- Blank Space -->
  
  <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">/s/&nbsp;Pamela Thomas-Graham</DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
  </TR>
  <TR valign="bottom">
      <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px"><DIV style="border-top: 1px solid #000000">Pamela Thomas-Graham</DIV>
  </DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">Director
  </TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">October&nbsp;7, 2005</TD>
  </TR>
  <TR valign="bottom">
  
  <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">&nbsp;</DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">&nbsp;</TD>
  </TR>
  <!-- End Table Body -->
  </TABLE>
  </DIV>
  
  
  <P align="center" style="font-size: 10pt">&nbsp;
  </DIV>
  
  <!-- PAGEBREAK -->
  <P><HR noshade><P>
  <H5 align="left" style="page-break-before:always"><A HREF="#toc">Table of Contents</A></H5><P>
  
  <DIV style="font-family: 'Times New Roman',Times,serif">
  
  
  <DIV align="center" style="font-size: 10pt; margin-top: 18pt"><B>INDEX TO EXHIBITS</B>
  </DIV>
  
  <DIV align="center">
  <TABLE style="font-size: 10pt" cellspacing="0" border="0" cellpadding="0" width="100%">
  <!-- Begin Table Head -->
  <TR valign="bottom">
      <TD width="8%">&nbsp;</TD>
      <TD width="2%">&nbsp;</TD>
      <TD width="90%">&nbsp;</TD>
  </TR>
  <TR style="font-size: 8pt" valign="bottom">
  
  <TD nowrap align="left">Number</TD>
      <TD>&nbsp;</TD>
      <TD nowrap align="left">Description</TD>
  </TR>
  
  <TR style="font-size:1px">
      <TD><DIV style="border-bottom:1px solid #000000; width=60%">&nbsp;</DIV></TD>
      <TD>&nbsp;</TD>
      <TD><DIV style="border-bottom:1px solid #000000; width=7.6%">&nbsp;</DIV></TD>
  </TR>
  
  
  <!-- End Table Head -->
  <!-- Begin Table Body -->
  <TR valign="bottom">
      <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">4.1(1)
  </DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">Restated Certificate of Incorporation of the Registrant, as amended</TD>
  </TR>
  <TR valign="bottom">
      <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">4.2(2)
  </DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">Amended and Restated By-Laws of the Registrant</TD>
  </TR>
  <TR valign="bottom">
      <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">5.1
  </DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">Opinion of Wilmer Cutler Pickering Hale and Dorr LLP, counsel to the
  Registrant</TD>
  </TR>
  <TR valign="bottom">
      <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">23.1
  </DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">Consent of Wilmer Cutler Pickering Hale and Dorr LLP
  (included in Exhibit&nbsp;5.1)</TD>
  </TR>
  <TR valign="bottom">
      <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">23.2
  </DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">Consent of PricewaterhouseCoopers LLP, independent registered public
  accounting firm</TD>
  </TR>
  <TR valign="bottom">
      <TD valign="top"><DIV style="margin-left:0px; text-indent:-0px">24
  </DIV></TD>
      <TD>&nbsp;</TD>
      <TD align="left" valign="top">Power of attorney (included on the signature pages of this
  registration statement)</TD>
  </TR>
  <!-- End Table Body -->
  </TABLE>
  </DIV>
  
  
  
  <DIV align="left">
  <DIV style="font-size: 3pt; margin-top: 16pt; width: 18%; border-top: 1px solid #000000">&nbsp;</DIV>
  </DIV>
  
  <TABLE width="100%" border="0" cellpadding="0" cellspacing="0" style="font-size: 10pt">
  <TR>
      <TD width="3%"></TD>
      <TD width="1%"></TD>
      <TD width="96"></TD>
  </TR>
  
  <TR valign="top">
      <TD nowrap align="left">(1)</TD>
      <TD>&nbsp;</TD>
      <TD>Previously filed with the Securities and Exchange Commission as an Exhibit to the
  Registrant&#146;s Registration Statement on Form S-1, as amended (File No.&nbsp;333-111157) and Form
  10-Q for the quarterly period ended June&nbsp;30, 2004 (File No.&nbsp;000-49839) and incorporated herein
  by reference.</TD>
  </TR>
  
  <TR style="font-size: 3pt"><TD>&nbsp;</TD></TR>
  
  <TR valign="top">
      <TD nowrap align="left">(2)</TD>
      <TD>&nbsp;</TD>
      <TD>Previously filed with the Securities and Exchange Commission as an Exhibit to the
  Registrant&#146;s Form 10-Q for the quarterly period ended June&nbsp;30, 2004 (File No.&nbsp;000-49839) and
  incorporated herein by reference.</TD>
  </TR>
  
  </TABLE>
  
  
  
  
  <P align="center" style="font-size: 10pt">&nbsp;
  </DIV>
  
  
  </BODY>
  </HTML>
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text: sv8
  Table of Contents
  As
  filed with the Securities and Exchange Commission on October 7, 2005
  Registration No. 333-_____
  SECURITIES AND EXCHANGE COMMISSION
  Washington, D.C. 20549
  FORM S-8
  REGISTRATION STATEMENT UNDER
  THE SECURITIES ACT OF 1933
  Idenix Pharmaceuticals, Inc.
  (Exact Name of Registrant as Specified in Its Charter)
  Delaware
  45-0478605
  (State or Other Jurisdiction of Incorporation
  (I.R.S. Employer
  or Organization)
  Identification No.)
  60 Hampshire Street
  02139
  Cambridge, MA
  (Zip Code)
  (Address of Principal Executive Offices)
  Idenix Pharmaceuticals, Inc.
  2005 Stock Incentive Plan
  (Full Title of the Plan)
  Jean-Pierre Sommadossi
  President and Chief Executive Officer
  Idenix Pharmaceuticals, Inc.
  60 Hampshire Street
  Cambridge, MA 02139
  (Name and Address of Agent For Service)
  617-995-9800
  (Telephone Number, Including Area Code, of Agent For Service)
  CALCULATION OF REGISTRATION FEE
  Proposed Maximum
  Proposed Maximum
  Amount to be
  Offering Price Per
  Aggregate Offering
  Amount of
  Title of Securities to be Registered
  Registered(1)
  Share
  Price
  Registration Fee
  Common Stock,
  $0.001 par value
  per share
  2,300,000 shares (2)
  $26.29 (3)
  $60,467,000 (3)
  $7,122
  (1)
  In accordance with Rule 416 under the Securities Act of 1933, as amended, this registration
  statement shall be deemed to cover any additional securities that may from time to time be
  offered or issued to prevent dilution resulting from stock splits, stock dividends or similar
  transactions.
  (2)
  Includes: (i) 8,000 shares of common stock originally reserved for issuance under the
  Registrant's 2004 Stock Incentive Plan (the "2004 Plan"), (ii) shares of common stock (not to
  exceed 792,000 additional shares) that are subject to awards granted under the 2004 Plan that
  expire, terminate or are otherwise cancelled without issuance of common stock; and (iii)
  1,500,000 shares of common stock originally reserved for issuance pursuant to the Registrant's
  2005 Stock Incentive Plan.
  (3)
  Estimated solely for the purpose of calculating the registration fee pursuant to Rules 457(c)
  and 457(h) of the Securities Act of 1933, as amended, and based upon the average of the high
  and low prices of the Registrant's Common Stock as reported on The NASDAQ National Market on
  October 5, 2005.
  TABLE OF CONTENTS
  PART I
  PART II
  SIGNATURES
  EX-5.1 Opinion of Wilmer Cutler Pickering Hale and Dorr LLP
  EX-23.2 Consent of PWC LLP
  Table of Contents
  PART I
  INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS
  Item 1. Plan Information.
  The information required by Item 1 is included in documents sent or given to participants in
  the plans covered by this registration statement pursuant to Rule 428(b)(1) of the Securities Act
  of 1933, as amended (the "Securities Act").
  Item 2. Registrant Information and Employee Plan Annual Information.
  The written statement required by Item 2 is included in documents sent or given to
  participants in the plans covered by this registration statement pursuant to Rule 428(b)(1) of the
  Securities Act.
  PART II
  INFORMATION REQUIRED IN THE REGISTRATION STATEMENT
  Item 3. Incorporation of Documents by Reference.
  The registrant is subject to the informational and reporting requirements of Sections 13(a),
  14, and 15(d) of the Securities Exchange Act of 1934, as amended (the "Exchange Act"), and in
  accordance therewith files reports, proxy statements and other information with the Securities and
  Exchange Commission (the "Commission"). The following documents, which are on file with the
  Commission, are incorporated in this registration statement by reference:
  (a) The registrant's latest annual report filed pursuant to Section 13(a) or 15(d) of the
  Exchange Act or the latest prospectus filed pursuant to Rule 424(b) under the Securities Act that
  contains audited financial statements for the registrant's latest fiscal year for which such
  statements have been filed.
  (b) All other reports filed pursuant to Section 13(a) or 15(d) of the Exchange Act since the
  end of the fiscal year covered by the document referred to in (a) above.
  (c) The description of the securities contained in the registrant's registration statements on
  Form 8-A filed under the Exchange Act, including any amendment or report filed for the purpose of
  updating such description.
  All documents subsequently filed by the registrant pursuant to Sections 13(a), 13(c), 14 and
  15(d) of the Exchange Act, prior to the filing of a post-effective amendment which indicates that
  all securities offered hereby have been sold or which deregisters all securities then remaining
  unsold, shall be deemed to be incorporated by reference in this registration statement and to be
  part hereof from the date of the filing of such documents. Any statement contained in a document
  incorporated or deemed to be incorporated by reference herein shall be deemed to be modified or
  superseded for the purposes of this registration statement to the extent that a statement contained
  herein or in any other subsequently filed document which also is or is deemed to be incorporated by
  reference herein modifies or supersedes such statement. Any statement so modified or superseded
  shall not be deemed, except as so modified or superseded, to constitute a part of this registration
  statement.
  Item 4. Description of Securities.
  Not applicable.
  Item 5. Interests of Named Experts and Counsel.
  Wilmer Cutler Pickering Hale and Dorr LLP has opined as to the legality of the securities
  being offered by this registration statement. Ambassador Charlene Barshefsky, a partner in the law
  firm of Wilmer Cutler Pickering Hale and Dorr LLP beneficially owns 61,343 shares of the
  registrant's common stock.
  Table of Contents
  Item 6. Indemnification of Directors and Officers.
  Section 145 of the General Corporation Law of the State of Delaware permits a corporation,
  under specified circumstances, to indemnify its directors, officers, employees or agents against
  expenses (including attorney's fees), judgments, fines and amounts paid in settlements actually and
  reasonably incurred by them in connection with any action, suit or proceeding brought by third
  parties by reason of the fact that they were or are directors, officers, employees or agents of the
  corporation, if such directors, officers, employees or agents acted in good faith and in a manner
  they reasonably believed to be in or not opposed to the best interests of the corporation and, with
  respect to any criminal action or proceeding, had no reason to believe their conduct was unlawful.
  In a derivative action, i.e., one by or in the right of the corporation, indemnification may be
  made only for expenses actually or reasonably incurred by directors, officers, employees or agents
  in connection with the defense or settlement of an action or suit, and only with respect to a
  matter as to which they shall have acted in good faith and in a manner they reasonably believed to
  be in or not opposed to the best interests of the corporation, except that no indemnification shall
  be made if such person shall have been adjudged liable to the corporation, unless and only to the
  extent that the court in which the action or suit was brought shall determine upon application that
  the defendant directors, officers, employees or agents are fairly and reasonably entitled to
  indemnify for such expenses despite such adjudication of liability.
  The registrant's restated certificate of incorporation, as amended, states that the
  registrant's directors will not have personal liability for monetary damages for any breach of
  fiduciary duty as a director, except to the extent that the General Corporation Law of the State of
  Delaware prohibits the elimination or limitation of liability of directors for breaches of
  fiduciary duty.
  The registrant also carries liability insurance covering each of its directors and officers.
  Item 7. Exemption from Registration Claimed.
  Not applicable.
  Item 8. Exhibits.
  The Exhibit Index immediately preceding the exhibits is incorporated herein by reference.
  Item 9. Undertakings.
  1.
  Item
  512(a)
  of Regulation S-K
  . The undersigned registrant hereby undertakes:
  (1)
  To file, during any period in which offers or sales are being made, a
  post-effective amendment to this registration statement:
  (i)
  To include any prospectus required by Section 10(a)(3) of the Securities Act;
  (ii)
  To reflect in the prospectus any facts or events arising after the effective date
  of the registration statement (or the most recent post-effective amendment thereof) which,
  individually or in the aggregate, represent a fundamental change in the information set
  forth in the registration statement; and
  (iii)
  To include any material information with respect to the plan of distribution not
  previously disclosed in the registration statement or any material change to such
  information in the registration statement;
  provided, however,
  that paragraphs (i) and (ii) do not apply if the information required to
  be included in a post-effective amendment by those paragraphs is contained in periodic
  reports filed with or furnished to the Commission by the registrant pursuant to Section 13
  or Section 15(d) of the Exchange Act that are incorporated by reference in the registration
  statement.
  Table of Contents
  (2)
  That, for the purpose of determining any liability under the Securities Act, each
  such post-effective amendment shall be deemed to be a new registration statement relating to
  the securities offered therein, and the offering of such securities at that time shall be
  deemed to be the initial
  bona fide
  offering thereof.
  (3)
  To remove from registration by means of a post-effective amendment any of the
  securities being registered which remain unsold at the termination of the offering.
  2.
  Item
  512(b)
  of Regulation S-K
  . The undersigned registrant hereby undertakes that,
  for purposes of determining any liability under the Securities Act, each filing of the registrant's
  annual report pursuant to Section 13(a) or Section 15(d) of the Exchange Act that is incorporated
  by reference in the registration statement shall be deemed to be a new registration statement
  relating to the securities offered therein, and the offering of such securities at that time shall
  be deemed to be the initial
  bona fide
  offering thereof.
  3.
  Item
  512(h)
  of Regulation S-K
  . Insofar as indemnification for liabilities arising
  under the Securities Act may be permitted to directors, officers and controlling persons of the
  registrant pursuant to the foregoing provisions, or otherwise, the registrant has been advised that
  in the opinion of the Securities and Exchange Commission such indemnification is against public
  policy as expressed in the Securities Act and is, therefore, unenforceable. In the event that a
  claim for indemnification against such liabilities (other than the payment by the registrant of
  expenses incurred or paid by a director, officer or controlling person of the registrant in the
  successful defense of any action, suit or proceeding) is asserted by such director, officer or
  controlling person in connection with the securities being registered, the registrant will, unless
  in the opinion of its counsel the matter has been settled by controlling precedent, submit to a
  court of appropriate jurisdiction the question whether such indemnification by it is against public
  policy as expressed in the Securities Act and will be governed by the final adjudication of such
  issue.
  Table of Contents
  SIGNATURES
  Pursuant to the requirements of the Securities Act of 1933, the registrant certifies that it
  has reasonable grounds to believe that it meets all of the requirements for filing on Form S-8 and
  has duly caused this registration statement to be signed on its behalf by the undersigned,
  thereunto duly authorized, in the City of Cambridge, Commonwealth of
  Massachusetts, on this 7
  th
  day
  of October, 2005.
  IDENIX PHARMACEUTICALS, INC.
  By:
  Jean-Pierre Sommadossi
  Jean-Pierre Sommadossi
  President and Chief Executive Officer
  POWER OF ATTORNEY AND SIGNATURES
  We, the undersigned officers and directors of Idenix Pharmaceuticals, Inc., hereby severally
  constitute and appoint Jean-Pierre Sommadossi and Andrea J. Corcoran, and each of them singly, our
  true and lawful attorneys with full power to them, and each of them singly, to sign for us and in
  our names in the capacities indicated below, the registration statement on Form S-8 filed herewith
  and any and all subsequent amendments to said registration statement, and generally to do all such
  things in our names and on our behalf in our capacities as officers and directors to enable Idenix
  Pharmaceuticals, Inc. to comply with the provisions of the Securities Act of 1933, as amended, and
  all requirements of the Securities and Exchange Commission, hereby ratifying and confirming our
  signatures as they may be signed by our said attorneys, or any of them, to said registration
  statement and any and all amendments thereto.
  Pursuant to the requirements of the Securities Act of 1933, this registration statement has
  been signed by the following persons in the capacities and on the dates indicated.
  Signature
  Title
  Date
  /s/ Jean-Pierre Sommadossi
  President and Chief Executive
  October 7, 2005
  Jean-Pierre Sommadossi
  Officer (Principal executive
  officer)
  /s/ David A. Arkowitz
  Chief Financial Officer and
  October 7, 2005
  David A. Arkowitz
  Treasurer (Principal financial
  and accounting officer)
  /s/ Charles W. Cramb
  Charles W. Cramb
  Director
  October 7, 2005
  /s/ Thomas Ebeling
  Thomas Ebeling
  Director
  October 7, 2005
  /s/ Wayne T. Hockmeyer
  Wayne T. Hockmeyer
  Director
  October 7, 2005
  /s/ Thomas R. Hodgson
  Thomas R. Hodgson
  Director
  October 7, 2005
  Table of Contents
  /s/ Robert
  E. Pelzer
  Robert E. Pelzer
  Director
  October 7, 2005
  /s/ Denise Pollard-Knight
  Denise Pollard-Knight
  Director
  October 7, 2005
  /s/ Pamela Thomas-Graham
  Pamela Thomas-Graham
  Director
  October 7, 2005
  Table of Contents
  INDEX TO EXHIBITS
  Number
  Description
  4.1(1)
  Restated Certificate of Incorporation of the Registrant, as amended
  4.2(2)
  Amended and Restated By-Laws of the Registrant
  5.1
  Opinion of Wilmer Cutler Pickering Hale and Dorr LLP, counsel to the
  Registrant
  23.1
  Consent of Wilmer Cutler Pickering Hale and Dorr LLP
  (included in Exhibit 5.1)
  23.2
  Consent of PricewaterhouseCoopers LLP, independent registered public
  accounting firm
  24
  Power of attorney (included on the signature pages of this
  registration statement)
  (1)
  Previously filed with the Securities and Exchange Commission as an Exhibit to the
  Registrant's Registration Statement on Form S-1, as amended (File No. 333-111157) and Form
  10-Q for the quarterly period ended June 30, 2004 (File No. 000-49839) and incorporated herein
  by reference.
  (2)
  Previously filed with the Securities and Exchange Commission as an Exhibit to the
  Registrant's Form 10-Q for the quarterly period ended June 30, 2004 (File No. 000-49839) and
  incorporated herein by reference.