content: <PAGE>
  
  As filed with the Securities and Exchange Commission on August 5, 2005
  
                                                        Registration No. 333-_____
  
  ================================================================================
  
                         SECURITIES AND EXCHANGE COMMISSION
                               WASHINGTON, D.C. 20549
  
                                      FORM S-8
  
                            REGISTRATION STATEMENT UNDER
                             THE SECURITIES ACT OF 1933
  
                                MKS INSTRUMENTS, INC.
               (Exact Name of Registrant as Specified in Its Charter)
  
           MASSACHUSETTS                                         04-2277512
  (State or Other Jurisdiction of                          (I.R.S. Employer
  Incorporation or Organization)                           Identification Number)
  
  90 INDUSTRIAL WAY, WILMINGTON, MASSACHUSETTS                     01887
  (Address of Principal Executive Offices)                       (Zip Code)
  
                              2004 STOCK INCENTIVE PLAN
                              (Full Title of the Plan)
  
                                  RONALD C. WEIGNER
                     VICE PRESIDENT AND CHIEF FINANCIAL OFFICER
                                MKS INSTRUMENTS, INC.
                                  90 INDUSTRIAL WAY
                                WILMINGTON, MA 01887
                       (Name and Address of Agent For Service)
  
                                   (978) 284-4000
            (Telephone Number, Including Area Code, of Agent For Service)
  
                           CALCULATION OF REGISTRATION FEE
  
  <TABLE>
  <CAPTION>
                                       Proposed Maximum    Proposed Maximum
  Title of Securities  Amount to be   Offering Price Per  Aggregate Offering     Amount of
   to be Registered    Registered(1)        Share               Price         Registration Fee
  -------------------  -------------  ------------------  ------------------  ----------------
  <S>                  <C>            <C>                 <C>                 <C>
  Common Stock, no      15,000,000        $18.83(2)         $282,450,000(2)       $33,245
  par value per share
  </TABLE>
  
  (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)   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
        August 2, 2005.
  
  <PAGE>
  
                                       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.
  
                                       - 2 -
  <PAGE>
  
        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.
  
        ITEM 6. INDEMNIFICATION OF DIRECTORS AND OFFICERS.
  
        Section 8.51(a) of Chapter 156D of the Massachusetts General Laws provides
  that a corporation may indemnify its directors against expenses (including
  attorneys' fees), judgments, fines and amounts paid in settlement reasonably
  incurred in connection with any litigation or other legal proceeding brought
  against any director by virtue of his position as a director of the registrant
  unless he is deemed to have not acted in good faith in the reasonable belief
  that his action was in the best interest of the corporation.
  
        Section 8.52 of Chapter 156D provides that a corporation must indemnify a
  director who is wholly successful, on the merits or otherwise, in the defense of
  any proceeding to which the director was a party because he was a director of
  the corporation against reasonable expenses incurred by him in connection with
  the proceeding.
  
        Section 8.56(a) of Chapter 156D provides that a corporation may indemnify
  its officers to the same extent as its directors and, for officers that are not
  directors, to the extent provided by (i) the articles of organization, (ii) the
  bylaws, (iii) a vote of the board of directors or (iv) a contract. In all
  instances, the extent to which a corporation provides indemnification to its
  officers under Section 8.56(a) is optional. Section 8.56(c) provides that an
  officer of a corporation who is not a director is entitled to mandatory
  indemnification under Section 8.52.
  
        In its Restated Articles of Organization (the "Articles of Organization"),
  the registrant has elected to commit to provide indemnification to its directors
  and officers in specified circumstances. Generally, Article 6 of the
  registrant's Articles of Organization provides that the registrant shall
  indemnify directors and officers of the registrant against liabilities and
  expenses arising out of legal proceedings brought against them by reason of
  their status as directors or officers or by reason of their agreeing to serve,
  at the request of the registrant, as a director or officer with another
  organization. Under this provision, a director or officer of the registrant
  shall be indemnified by the registrant for all expenses (including attorneys'
  fees), judgments and fines, and amounts paid in settlement of such proceedings,
  even if he is not successful on the merits, unless finally adjudicated not to
  have acted in good faith in the reasonable belief that his action was in the
  best interests of the registrant. The registrant shall not indemnify an officer
  or director who initiates the legal proceeding, unless the initiation was
  approved by the Board of Directors. The Board of Directors may authorize
  advancing litigation expenses to a director or officer at his request upon
  receipt of an undertaking by any such director or officer to repay such expenses
  if it is ultimately determined that he is not entitled to indemnification for
  such expenses.
  
        Section 2.02(b)(4) of Chapter 156D of the Massachusetts General Laws
  allows a corporation to eliminate or limit the personal liability of a director
  of a corporation to the corporation for monetary damages for a breach of
  fiduciary duty as a director notwithstanding any provision of law imposing such
  liability, except where the director breached his duty of loyalty to the
  corporation or its shareholders, failed to act in good faith, engaged in
  intentional misconduct or knowingly violated a law, authorized the payment of an
  improper distribution or obtained an improper personal benefit. Article 6A of
  the registrant's Articles of Organization eliminates the personal liability of
  the registrant's directors to the registrant or its stockholders for monetary
  damages for breach of a director's fiduciary duty, except to the extent Chapter
  156D of the Massachusetts General Laws prohibits the elimination or limitation
  of such liability.
  
                                       - 3 -
  <PAGE>
  
        The registrant has obtained directors and officers' liability insurance
  for the benefit of its directors and certain of its 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.
  
              (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.
  
                                       - 4 -
  <PAGE>
  
        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.
  
                                       - 5 -
  <PAGE>
  
                                     SIGNATURES
  
        Pursuant to the requirements of the Securities Act, 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 Wilmington, Massachusetts on August 1, 2005.
  
                               MKS INSTRUMENTS, INC.
  
                               By: /s/ Leo Berlinghieri
                                   ------------------------
                                   Leo Berlinghieri
                                   President, Chief Executive Officer and Director
  
                          POWER OF ATTORNEY AND SIGNATURES
  
        We, the undersigned officers and directors of MKS Instruments, Inc. hereby
  severally constitute and appoint Leo Berlinghieri, Ronald C. Weigner and
  Kathleen F. Burke, 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 behalf in our capacities as
  officers and directors to enable MKS Instruments, Inc. to comply with all
  requirements of the Securities and Exchange Commission, hereby ratifying and
  confirming our signatures as they may be signed by said attorneys, or any of
  them, to said Registration Statement and any and all amendments thereto.
  
        Pursuant to the requirements of the Securities Act, this Registration
  Statement has been signed by the following persons in the capacities and on the
  dates indicated.
  
  <TABLE>
  <CAPTION>
       SIGNATURE                   TITLE                        DATE
       ---------                   -----                        ----
  <S>                     <C>                              <C>
                          President, Chief Executive       August 1, 2005
  /s/ Leo Berlinghieri    Officer and Director (Principal
  ----------------------  Executive Officer)
  Leo Berlinghieri
  
  /s/ Ronald C. Weigner   Vice President and Chief         August 1, 2005
  ----------------------  Financial Officer (Principal
  Ronald C. Weigner       Financial Officer and Principal
                          Accounting Officer)
  
  /s/ John R. Bertucci    Director                         August 1, 2005
  ----------------------
  John R. Bertucci
  
  /s/ Robert R. Anderson  Director                         August 1, 2005
  ----------------------
  Robert R. Anderson
  
  /s/ James G. Berges     Director                         August 1, 2005
  ----------------------
  James G. Berges
  
  /s/ Richard S. Chute    Director                         August 1, 2005
  ----------------------
  Richard S. Chute
  
  /s/ Hans-Jochen Kahl    Director                         August 1, 2005
  ----------------------
  Hans-Jochen Kahl
  
  /s/ Owen W. Robbins     Director                         August 1, 2005
  ----------------------
  Owen W. Robbins
  
  /s/ Louis P. Valente    Director                         August 1, 2005
  ----------------------
  Louis P. Valente
  </TABLE>
  
                                       - 6 -
  <PAGE>
  
                                    EXHIBIT INDEX
  
  <TABLE>
  <CAPTION>
   Exhibit
   Number                     Description
   -------                    -----------
  <S>       <C>
   4.1 (1)  Restated Articles of Organization of the Registrant.
  
   4.2 (2)  Articles of Amendment.
  
   4.3 (3)  Articles of Amendment.
  
   4.4 (4)  Amended and Restated By-Laws of the Registrant.
  
   4.5 (4)  Specimen Certificate for Common Stock of the Registrant.
  
   5.1      Opinion of Wilmer Cutler Pickering Hale and Dorr LLP.
  
   23.1     Consent of Wilmer Cutler Pickering Hale and Dorr LLP (included in
            Exhibit 5.1).
  
   23.2     Consent of PricewaterhouseCoopers LLP.
  
   24.1     Power of Attorney (included in the signature pages of this
            Registration Statement).
  </TABLE>
  
  ----------------
  
  (1)   Incorporated herein by reference from the Registrant's Registration
        Statement on Form S-4 (File No. 333-49738) filed with the Securities and
        Exchange Commission on November 13, 2000.
  
  (2)   Incorporated herein by reference from the Registrant's Quarterly Report on
        Form 10-Q (File No. 000-23621) for the quarter ended June 30, 2001.
  
  (3)   Incorporated herein by reference from the Registrant's Quarterly Report on
        Form 10-Q (File No. 000-23621) for the quarter ended June 30, 2002.
  
  (4)   Incorporated herein by reference from the Registrant's Registration
        Statement on Form S-1 (File No. 333-71363) filed with the Securities and
        Exchange Commission on January 28, 1999, as amended.
  
                                       - 7 -
metadata_documents: [{"type": "S-8", "sequence": "1", "filename": "b56147misv8.txt", "description": "MKS INSTRUMENTS, INC.", "secsgml_size_bytes": 19041}]
metadata_filer: {"company-data": {"conformed-name": "MKS INSTRUMENTS INC", "cik": "0001049502", "assigned-sic": "3823", "irs-number": "042277512", "state-of-incorporation": "MA", "fiscal-year-end": "1231"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-127221", "film-number": "051001042"}, "business-address": {"street1": "90 INDUSTRIAL WAY", "city": "WILMINGTON", "state": "MA", "zip": "01887", "phone": "978-284-4000"}, "mail-address": {"street1": "90 INDUSTRIAL WAY", "city": "WILMINGTON", "state": "MA", "zip": "01887"}}
text: As filed with the Securities and Exchange Commission on August 5, 2005
  
                                                        Registration No. 333-_____
  
  ================================================================================
  
                         SECURITIES AND EXCHANGE COMMISSION
                               WASHINGTON, D.C. 20549
  
                                      FORM S-8
  
                            REGISTRATION STATEMENT UNDER
                             THE SECURITIES ACT OF 1933
  
                                MKS INSTRUMENTS, INC.
               (Exact Name of Registrant as Specified in Its Charter)
  
           MASSACHUSETTS                                         04-2277512
  (State or Other Jurisdiction of                          (I.R.S. Employer
  Incorporation or Organization)                           Identification Number)
  
  90 INDUSTRIAL WAY, WILMINGTON, MASSACHUSETTS                     01887
  (Address of Principal Executive Offices)                       (Zip Code)
  
                              2004 STOCK INCENTIVE PLAN
                              (Full Title of the Plan)
  
                                  RONALD C. WEIGNER
                     VICE PRESIDENT AND CHIEF FINANCIAL OFFICER
                                MKS INSTRUMENTS, INC.
                                  90 INDUSTRIAL WAY
                                WILMINGTON, MA 01887
                       (Name and Address of Agent For Service)
  
                                   (978) 284-4000
            (Telephone Number, Including Area Code, of Agent For Service)
  
                           CALCULATION OF REGISTRATION FEE
  Proposed Maximum    Proposed Maximum
  Title of Securities  Amount to be   Offering Price Per  Aggregate Offering     Amount of
   to be Registered    Registered(1)        Share               Price         Registration Fee
  -------------------  -------------  ------------------  ------------------  ----------------
  Common Stock, no      15,000,000        $18.83(2)         $282,450,000(2)       $33,245
  par value per share
  (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)   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
        August 2, 2005.
  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.
  
                                       - 2 -
  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.
  
        ITEM 6. INDEMNIFICATION OF DIRECTORS AND OFFICERS.
  
        Section 8.51(a) of Chapter 156D of the Massachusetts General Laws provides
  that a corporation may indemnify its directors against expenses (including
  attorneys' fees), judgments, fines and amounts paid in settlement reasonably
  incurred in connection with any litigation or other legal proceeding brought
  against any director by virtue of his position as a director of the registrant
  unless he is deemed to have not acted in good faith in the reasonable belief
  that his action was in the best interest of the corporation.
  
        Section 8.52 of Chapter 156D provides that a corporation must indemnify a
  director who is wholly successful, on the merits or otherwise, in the defense of
  any proceeding to which the director was a party because he was a director of
  the corporation against reasonable expenses incurred by him in connection with
  the proceeding.
  
        Section 8.56(a) of Chapter 156D provides that a corporation may indemnify
  its officers to the same extent as its directors and, for officers that are not
  directors, to the extent provided by (i) the articles of organization, (ii) the
  bylaws, (iii) a vote of the board of directors or (iv) a contract. In all
  instances, the extent to which a corporation provides indemnification to its
  officers under Section 8.56(a) is optional. Section 8.56(c) provides that an
  officer of a corporation who is not a director is entitled to mandatory
  indemnification under Section 8.52.
  
        In its Restated Articles of Organization (the "Articles of Organization"),
  the registrant has elected to commit to provide indemnification to its directors
  and officers in specified circumstances. Generally, Article 6 of the
  registrant's Articles of Organization provides that the registrant shall
  indemnify directors and officers of the registrant against liabilities and
  expenses arising out of legal proceedings brought against them by reason of
  their status as directors or officers or by reason of their agreeing to serve,
  at the request of the registrant, as a director or officer with another
  organization. Under this provision, a director or officer of the registrant
  shall be indemnified by the registrant for all expenses (including attorneys'
  fees), judgments and fines, and amounts paid in settlement of such proceedings,
  even if he is not successful on the merits, unless finally adjudicated not to
  have acted in good faith in the reasonable belief that his action was in the
  best interests of the registrant. The registrant shall not indemnify an officer
  or director who initiates the legal proceeding, unless the initiation was
  approved by the Board of Directors. The Board of Directors may authorize
  advancing litigation expenses to a director or officer at his request upon
  receipt of an undertaking by any such director or officer to repay such expenses
  if it is ultimately determined that he is not entitled to indemnification for
  such expenses.
  
        Section 2.02(b)(4) of Chapter 156D of the Massachusetts General Laws
  allows a corporation to eliminate or limit the personal liability of a director
  of a corporation to the corporation for monetary damages for a breach of
  fiduciary duty as a director notwithstanding any provision of law imposing such
  liability, except where the director breached his duty of loyalty to the
  corporation or its shareholders, failed to act in good faith, engaged in
  intentional misconduct or knowingly violated a law, authorized the payment of an
  improper distribution or obtained an improper personal benefit. Article 6A of
  the registrant's Articles of Organization eliminates the personal liability of
  the registrant's directors to the registrant or its stockholders for monetary
  damages for breach of a director's fiduciary duty, except to the extent Chapter
  156D of the Massachusetts General Laws prohibits the elimination or limitation
  of such liability.
  
                                       - 3 -
  The registrant has obtained directors and officers' liability insurance
  for the benefit of its directors and certain of its 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.
  
              (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.
  
                                       - 4 -
  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.
  
                                       - 5 -
  SIGNATURES
  
        Pursuant to the requirements of the Securities Act, 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 Wilmington, Massachusetts on August 1, 2005.
  
                               MKS INSTRUMENTS, INC.
  
                               By: /s/ Leo Berlinghieri
                                   ------------------------
                                   Leo Berlinghieri
                                   President, Chief Executive Officer and Director
  
                          POWER OF ATTORNEY AND SIGNATURES
  
        We, the undersigned officers and directors of MKS Instruments, Inc. hereby
  severally constitute and appoint Leo Berlinghieri, Ronald C. Weigner and
  Kathleen F. Burke, 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 behalf in our capacities as
  officers and directors to enable MKS Instruments, Inc. to comply with all
  requirements of the Securities and Exchange Commission, hereby ratifying and
  confirming our signatures as they may be signed by said attorneys, or any of
  them, to said Registration Statement and any and all amendments thereto.
  
        Pursuant to the requirements of the Securities Act, this Registration
  Statement has been signed by the following persons in the capacities and on the
  dates indicated.
  SIGNATURE                   TITLE                        DATE
       ---------                   -----                        ----
  President, Chief Executive       August 1, 2005
  /s/ Leo Berlinghieri    Officer and Director (Principal
  ----------------------  Executive Officer)
  Leo Berlinghieri
  
  /s/ Ronald C. Weigner   Vice President and Chief         August 1, 2005
  ----------------------  Financial Officer (Principal
  Ronald C. Weigner       Financial Officer and Principal
                          Accounting Officer)
  
  /s/ John R. Bertucci    Director                         August 1, 2005
  ----------------------
  John R. Bertucci
  
  /s/ Robert R. Anderson  Director                         August 1, 2005
  ----------------------
  Robert R. Anderson
  
  /s/ James G. Berges     Director                         August 1, 2005
  ----------------------
  James G. Berges
  
  /s/ Richard S. Chute    Director                         August 1, 2005
  ----------------------
  Richard S. Chute
  
  /s/ Hans-Jochen Kahl    Director                         August 1, 2005
  ----------------------
  Hans-Jochen Kahl
  
  /s/ Owen W. Robbins     Director                         August 1, 2005
  ----------------------
  Owen W. Robbins
  
  /s/ Louis P. Valente    Director                         August 1, 2005
  ----------------------
  Louis P. Valente
  - 6 -
  EXHIBIT INDEX
  Exhibit
   Number                     Description
   -------                    -----------
  4.1 (1)  Restated Articles of Organization of the Registrant.
  
   4.2 (2)  Articles of Amendment.
  
   4.3 (3)  Articles of Amendment.
  
   4.4 (4)  Amended and Restated By-Laws of the Registrant.
  
   4.5 (4)  Specimen Certificate for Common Stock of the Registrant.
  
   5.1      Opinion of Wilmer Cutler Pickering Hale and Dorr LLP.
  
   23.1     Consent of Wilmer Cutler Pickering Hale and Dorr LLP (included in
            Exhibit 5.1).
  
   23.2     Consent of PricewaterhouseCoopers LLP.
  
   24.1     Power of Attorney (included in the signature pages of this
            Registration Statement).
  ----------------
  
  (1)   Incorporated herein by reference from the Registrant's Registration
        Statement on Form S-4 (File No. 333-49738) filed with the Securities and
        Exchange Commission on November 13, 2000.
  
  (2)   Incorporated herein by reference from the Registrant's Quarterly Report on
        Form 10-Q (File No. 000-23621) for the quarter ended June 30, 2001.
  
  (3)   Incorporated herein by reference from the Registrant's Quarterly Report on
        Form 10-Q (File No. 000-23621) for the quarter ended June 30, 2002.
  
  (4)   Incorporated herein by reference from the Registrant's Registration
        Statement on Form S-1 (File No. 333-71363) filed with the Securities and
        Exchange Commission on January 28, 1999, as amended.
  
                                       - 7 -