content: <PAGE>   1
      As filed with the Securities and Exchange Commission on January 29, 2001
                                                     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)
  
             SIX SHATTUCK ROAD                                  01810
           ANDOVER, MASSACHUSETTS                             (Zip Code)
  (Address of Principal Executive Offices)
  
  
                           1994 FORMULA STOCK OPTION PLAN
                       OF APPLIED SCIENCE AND TECHNOLOGY, INC.
                              (Full Title of the Plan)
  
                                  JOHN R. BERTUCCI
                        CHAIRMAN AND CHIEF EXECUTIVE OFFICER
                                MKS INSTRUMENTS, INC.
                                  SIX SHATTUCK ROAD
                                  ANDOVER, MA 01810
                       (Name and Address of Agent for Service)
  
                                   (978) 975-2350
            (Telephone Number, Including Area Code, of Agent for Service)
  
                           CALCULATION OF REGISTRATION FEE
  
  <TABLE>
  <CAPTION>
  --------------------------------------------------------------------------------------------------------------------
                                                       Proposed              Proposed
            Title of                                    Maximum               Maximum
           Securities               Amount             Offering              Aggregate               Amount of
             to be                  to be                Price                Offering              Registration
           Registered             Registered(1)        Per Share               Price(2)                 Fee
  ====================================================================================================================
  
  <S>                             <C>                  <C>                  <C>                     <C>
  Common Stock no par value         138,042              $21.25               $2,933,393                $734
  
  --------------------------------------------------------------------------------------------------------------------
  </TABLE>
  
  (1)  Based on 180,000 shares of ASTeX common stock subject to outstanding
       options under the 1994 Formula Stock Option Plan of ASTeX. Shares
       underlying ASTeX options have been converted to MKS shares based on an
       exchange ratio of .7669.
  
  (2)  Estimated solely for the purpose of calculating the registration fee, and
       based on the weighted average exercise price of the ASTeX options in
       accordance with Rule 457(h) under the Securities Act of 1933, as amended.
       The weighted average exercise price of the ASTeX options has been converted
       using the exchange ratio of .7669.
  
  
  <PAGE>   2
  
  
  PART I. INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS
  
       The information required by Part I is included in documents sent or given
  to participants in the 1994 Formula Stock Option Plan pursuant to Rule 428(b)(1)
  of the Securities Act of 1933, as amended (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:
  
            (1)  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.
  
            (2)  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 (1) above.
  
            (3)  The description of the common stock of the Registrant, no par
                 value per share (the "Common Stock"), contained in a registration
                 statement 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 shares of Common Stock offered
  hereby have been sold or which deregisters all shares of Common Stock then
  remaining unsold, shall be deemed to be incorporated by reference herein 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 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.
  
  
  <PAGE>   3
  
  
  Item 5. INTERESTS OF NAMED EXPERTS AND COUNSEL
  
       Not applicable.
  
  Item 6. INDEMNIFICATION OF DIRECTORS AND OFFICERS
  
       The Registrant is a Massachusetts corporation. Reference is made to Chapter
  156B, Section 13 of the Massachusetts Business Corporation Law (the "MBCL"),
  which enables a corporation in its original articles of organization or an
  amendment thereto to eliminate or limit the personal liability of a director for
  monetary damages for violations of the director's fiduciary duty, except (i) for
  any breach of the director's duty of loyalty to the corporation or its
  stockholders, (ii) for acts or omissions not in good faith or which involve
  intentional misconduct or a knowing violation of law, (iii) pursuant to Sections
  61 and 62 of the MBCL (providing for liability of directors for authorizing
  illegal distributions and for making loans to directors, officers and certain
  stockholders) or (iv) for any transaction from which a director derived an
  improper personal benefit.
  
       Reference also is made to Chapter 156B, Section 67 of the MBCL, which
  provides that a corporation may indemnify directors, officers, employees and
  other agents and persons who serve at its request as directors, officers,
  employees or other agents of another organization or who serve at its request in
  any capacity with respect to any employee benefit plan, to the extent specified
  or authorized by the articles of organization, a by-law adopted by the
  stockholders or a vote adopted by the holders of a majority of the shares of
  stock entitled to vote on the election of directors. Such indemnification may
  include payment by the corporation of expenses incurred in defending a civil or
  criminal action or proceeding in advance of the final disposition of such action
  or proceeding, upon receipt of an undertaking by the person indemnified to repay
  such payment if he shall be adjudicated to be not entitled to indemnification
  under Section 67, which undertaking may be accepted without reference to the
  financial ability of such person to make repayment. Any such indemnification may
  be provided although the person to be indemnified is no longer an officer,
  director, employee or agent of the corporation or of such other organization or
  no longer serves with respect to any such employee benefit plan. No
  indemnification shall be provided, however, for any person with respect to any
  matter as to which he shall have been adjudicated in any proceeding not to have
  acted in good faith in the reasonable belief that his action was in the best
  interest of the corporation or to the extent that such matter relates to service
  with respect to any employee benefit plan, in the best interests of the
  participants or beneficiaries of such employee benefit plan.
  
       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 costs and expenses (including
  attorneys' fees), judgments, liabilities and amounts paid in settlement of such
  proceedings, even if he is not successful on the merits, if he acted in good
  faith in the reasonable belief that his action was in
  
  
  <PAGE>   4
  
  
  the best interests of the Registrant. 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.
  
       Article 6 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 156B of the Massachusetts General Laws prohibits
  the elimination or limitation of such liability.
  
       The Registrant has obtained directors and officers liability insurance for
  the benefit of its directors an
  
  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.   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. Notwithstanding the foregoing, any increase or
                      decrease in the volume of securities offered (if the total
                      dollar value of securities offered would not exceed that
                      which was registered) and any deviation from the low or high
                      end of the estimated maximum offering rage may be reflected
                      in the form of prospectus filed with the Commission pursuant
                      to Rule 424(b) if, in the aggregate, the changes in volume
                      and price represent no more than 20 percent change in the
                      maximum offering price set forth in the "Calculation of
                      Registration Fee" table in the effective Registration
                      Statement;
  
  
  <PAGE>   5
  
  
                 (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.
  
            (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.   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 15(d) of the Exchange
  Act (and, where applicable, each filing of an employee benefit plan's annual
  report pursuant to 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.   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 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.
  
  
  <PAGE>   6
  
  
                                     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 Andover, Massachusetts on January 25, 2001.
  
                                               MKS INSTRUMENTS, INC.
  
  
                                               By: /s/ John R. Bertucci
                                                   -----------------------
                                                   John R. Bertucci
                                                   Chairman of the Board and Chief
                                                     Executive Officer
  
  
                          POWER OF ATTORNEY AND SIGNATURES
  
       We, the undersigned officers and directors of MKS Instruments, Inc. hereby
  severally constitute and appoint John R. Bertucci, Ronald C. Weigner and Mark G.
  Borden, 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>
  /s/ John R. Bertucci                            Chairman of the Board and Chief     January 25, 2001
  ------------------------                        Executive Officer (Principal
  John R. Bertucci                                Executive Officer)
  
  /s/ Ronald C. Weigner                           Vice President and Chief            January 25, 2001
  ------------------------                        Financial Officer (Principal
  Ronald C. Weigner                               Financial Officer and Principal
                                                  Accounting Officer)
  
  /s/ Richard S. Chute                            Director                            January 25, 2001
  ------------------------
  Richard S. Chute
  
  
  /s/ Owen W. Robbins                             Director                            January 25, 2001
  ------------------------
  Owen W. Robbins
  
  
  /s/ Robert J. Therrien                          Director                            January 25, 2001
  ------------------------
  Robert J. Therrien
  
  
  /s/ Louis P. Valente                            Director                            January 25, 2001
  ------------------------
  Louis P. Valente
  
  </TABLE>
  
  
  <PAGE>   7
  
  
                                    EXHIBIT INDEX
  
  
  <TABLE>
  <CAPTION>
   Exhibit
   Number                             Description
   -------                            -----------
  
  <S>            <C>
  4.1 (1)        Restated Articles of Organization of the Registrant.
  
  4.2 (2)        Amended and Restated By-Laws of the Registrant.
  
  4.3 (2)        Specimen Certificate for Common Stock of the Registrant.
  
  4.4            Applied Science and Technology, Inc. 1994 Formula Stock Option Plan.
  
  5              Opinion of Hale and Dorr LLP.
  
  23.1           Consent of Hale and Dorr LLP (included in Exhibit 5).
  
  23.2           Consent of PricewaterhouseCoopers LLP.
  
  24             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) originally filed with the
       Securities and Exchange Commission on November 13, 2000, as amended.
  
  (2)  Incorporated herein by reference from the Registrant's Registration
       Statement on Form S-1 (File No. 333-71363) originally filed with the
       Securities and Exchange Commission on January 28, 1999, as amended.
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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": "333-54488", "film-number": "1517217"}, "business-address": {"street1": "SIX SHATTUCK RD", "city": "ANDOVER", "state": "MA", "zip": "01810", "phone": "9789752350"}, "mail-address": {"street1": "SIX SHATTUCK ROAD", "street2": "ATTN: CORPORATE ACCOUNTING", "city": "ANDOVER", "state": "MA", "zip": "01810"}}
text: 1
      As filed with the Securities and Exchange Commission on January 29, 2001
                                                     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)
  
             SIX SHATTUCK ROAD                                  01810
           ANDOVER, MASSACHUSETTS                             (Zip Code)
  (Address of Principal Executive Offices)
  
                           1994 FORMULA STOCK OPTION PLAN
                       OF APPLIED SCIENCE AND TECHNOLOGY, INC.
                              (Full Title of the Plan)
  
                                  JOHN R. BERTUCCI
                        CHAIRMAN AND CHIEF EXECUTIVE OFFICER
                                MKS INSTRUMENTS, INC.
                                  SIX SHATTUCK ROAD
                                  ANDOVER, MA 01810
                       (Name and Address of Agent for Service)
  
                                   (978) 975-2350
            (Telephone Number, Including Area Code, of Agent for Service)
  
                           CALCULATION OF REGISTRATION FEE
  --------------------------------------------------------------------------------------------------------------------
                                                       Proposed              Proposed
            Title of                                    Maximum               Maximum
           Securities               Amount             Offering              Aggregate               Amount of
             to be                  to be                Price                Offering              Registration
           Registered             Registered(1)        Per Share               Price(2)                 Fee
  ====================================================================================================================
  Common Stock no par value         138,042              $21.25               $2,933,393                $734
  
  --------------------------------------------------------------------------------------------------------------------
  (1)  Based on 180,000 shares of ASTeX common stock subject to outstanding
       options under the 1994 Formula Stock Option Plan of ASTeX. Shares
       underlying ASTeX options have been converted to MKS shares based on an
       exchange ratio of .7669.
  
  (2)  Estimated solely for the purpose of calculating the registration fee, and
       based on the weighted average exercise price of the ASTeX options in
       accordance with Rule 457(h) under the Securities Act of 1933, as amended.
       The weighted average exercise price of the ASTeX options has been converted
       using the exchange ratio of .7669.
  2
  
  PART I. INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS
  
       The information required by Part I is included in documents sent or given
  to participants in the 1994 Formula Stock Option Plan pursuant to Rule 428(b)(1)
  of the Securities Act of 1933, as amended (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:
  
            (1)  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.
  
            (2)  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 (1) above.
  
            (3)  The description of the common stock of the Registrant, no par
                 value per share (the "Common Stock"), contained in a registration
                 statement 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 shares of Common Stock offered
  hereby have been sold or which deregisters all shares of Common Stock then
  remaining unsold, shall be deemed to be incorporated by reference herein 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 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.
  3
  
  Item 5. INTERESTS OF NAMED EXPERTS AND COUNSEL
  
       Not applicable.
  
  Item 6. INDEMNIFICATION OF DIRECTORS AND OFFICERS
  
       The Registrant is a Massachusetts corporation. Reference is made to Chapter
  156B, Section 13 of the Massachusetts Business Corporation Law (the "MBCL"),
  which enables a corporation in its original articles of organization or an
  amendment thereto to eliminate or limit the personal liability of a director for
  monetary damages for violations of the director's fiduciary duty, except (i) for
  any breach of the director's duty of loyalty to the corporation or its
  stockholders, (ii) for acts or omissions not in good faith or which involve
  intentional misconduct or a knowing violation of law, (iii) pursuant to Sections
  61 and 62 of the MBCL (providing for liability of directors for authorizing
  illegal distributions and for making loans to directors, officers and certain
  stockholders) or (iv) for any transaction from which a director derived an
  improper personal benefit.
  
       Reference also is made to Chapter 156B, Section 67 of the MBCL, which
  provides that a corporation may indemnify directors, officers, employees and
  other agents and persons who serve at its request as directors, officers,
  employees or other agents of another organization or who serve at its request in
  any capacity with respect to any employee benefit plan, to the extent specified
  or authorized by the articles of organization, a by-law adopted by the
  stockholders or a vote adopted by the holders of a majority of the shares of
  stock entitled to vote on the election of directors. Such indemnification may
  include payment by the corporation of expenses incurred in defending a civil or
  criminal action or proceeding in advance of the final disposition of such action
  or proceeding, upon receipt of an undertaking by the person indemnified to repay
  such payment if he shall be adjudicated to be not entitled to indemnification
  under Section 67, which undertaking may be accepted without reference to the
  financial ability of such person to make repayment. Any such indemnification may
  be provided although the person to be indemnified is no longer an officer,
  director, employee or agent of the corporation or of such other organization or
  no longer serves with respect to any such employee benefit plan. No
  indemnification shall be provided, however, for any person with respect to any
  matter as to which he shall have been adjudicated in any proceeding not to have
  acted in good faith in the reasonable belief that his action was in the best
  interest of the corporation or to the extent that such matter relates to service
  with respect to any employee benefit plan, in the best interests of the
  participants or beneficiaries of such employee benefit plan.
  
       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 costs and expenses (including
  attorneys' fees), judgments, liabilities and amounts paid in settlement of such
  proceedings, even if he is not successful on the merits, if he acted in good
  faith in the reasonable belief that his action was in
  4
  
  the best interests of the Registrant. 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.
  
       Article 6 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 156B of the Massachusetts General Laws prohibits
  the elimination or limitation of such liability.
  
       The Registrant has obtained directors and officers liability insurance for
  the benefit of its directors an
  
  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.   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. Notwithstanding the foregoing, any increase or
                      decrease in the volume of securities offered (if the total
                      dollar value of securities offered would not exceed that
                      which was registered) and any deviation from the low or high
                      end of the estimated maximum offering rage may be reflected
                      in the form of prospectus filed with the Commission pursuant
                      to Rule 424(b) if, in the aggregate, the changes in volume
                      and price represent no more than 20 percent change in the
                      maximum offering price set forth in the "Calculation of
                      Registration Fee" table in the effective Registration
                      Statement;
  5
  
                 (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.
  
            (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.   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 15(d) of the Exchange
  Act (and, where applicable, each filing of an employee benefit plan's annual
  report pursuant to 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.   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 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.
  6
  
                                     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 Andover, Massachusetts on January 25, 2001.
  
                                               MKS INSTRUMENTS, INC.
  
                                               By: /s/ John R. Bertucci
                                                   -----------------------
                                                   John R. Bertucci
                                                   Chairman of the Board and Chief
                                                     Executive Officer
  
                          POWER OF ATTORNEY AND SIGNATURES
  
       We, the undersigned officers and directors of MKS Instruments, Inc. hereby
  severally constitute and appoint John R. Bertucci, Ronald C. Weigner and Mark G.
  Borden, 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
       ---------                                           -----                           ----
  /s/ John R. Bertucci                            Chairman of the Board and Chief     January 25, 2001
  ------------------------                        Executive Officer (Principal
  John R. Bertucci                                Executive Officer)
  
  /s/ Ronald C. Weigner                           Vice President and Chief            January 25, 2001
  ------------------------                        Financial Officer (Principal
  Ronald C. Weigner                               Financial Officer and Principal
                                                  Accounting Officer)
  
  /s/ Richard S. Chute                            Director                            January 25, 2001
  ------------------------
  Richard S. Chute
  
  /s/ Owen W. Robbins                             Director                            January 25, 2001
  ------------------------
  Owen W. Robbins
  
  /s/ Robert J. Therrien                          Director                            January 25, 2001
  ------------------------
  Robert J. Therrien
  
  /s/ Louis P. Valente                            Director                            January 25, 2001
  ------------------------
  Louis P. Valente
  7
  
                                    EXHIBIT INDEX
  Exhibit
   Number                             Description
   -------                            -----------
  4.1 (1)        Restated Articles of Organization of the Registrant.
  
  4.2 (2)        Amended and Restated By-Laws of the Registrant.
  
  4.3 (2)        Specimen Certificate for Common Stock of the Registrant.
  
  4.4            Applied Science and Technology, Inc. 1994 Formula Stock Option Plan.
  
  5              Opinion of Hale and Dorr LLP.
  
  23.1           Consent of Hale and Dorr LLP (included in Exhibit 5).
  
  23.2           Consent of PricewaterhouseCoopers LLP.
  
  24             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) originally filed with the
       Securities and Exchange Commission on November 13, 2000, as amended.
  
  (2)  Incorporated herein by reference from the Registrant's Registration
       Statement on Form S-1 (File No. 333-71363) originally filed with the
       Securities and Exchange Commission on January 28, 1999, as amended.