content: <PAGE>   1
  
  
     As filed with the Securities and Exchange Commission on February 28, 2000
  
                                                        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
           (State or Other Jurisdiction of Incorporation or Organization)
  
                                     04-2277512
                        (I.R.S. Employer Identification No.)
  
  SIX SHATTUCK ROAD, ANDOVER, MASSACHUSETTS                        01810
   (Address of Principal Executive Offices)                      (Zip Code)
  
                         MKS INSTRUMENTS, INC. INTERNATIONAL
                            EMPLOYEE STOCK PURCHASE PLAN
                              (Full Title of the Plan)
  
                                  JOHN R. BERTUCCI
                                MKS INSTRUMENTS, INC.
                                  SIX SHATTUCK ROAD
                            ANDOVER, MASSACHUSETTS 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
        TITLE OF                                        MAXIMUM
       SECURITIES                                      OFFERING           PROPOSED MAXIMUM          AMOUNT OF
          TO BE                   AMOUNT               PRICE PER         AGGREGATE OFFERING       REGISTRATION
       REGISTERED            TO BE REGISTERED            SHARE                 PRICE                    FEE
  ------------------------------------------------------------------------------------------------------------
  <S>                            <C>                   <C>                 <C>                       <C>
  Common Stock,
  no par value                   50,000                $43.56(1)           $2,178,000(1)             $574.99
  ------------------------------------------------------------------------------------------------------------
  </TABLE>
  
  (1)        Estimated solely for the purpose of calculating the registration fee,
             and based upon the average of the high and low prices of the Common
             Stock on the Nasdaq National Market on February 24, 2000 in
             accordance with Rules 457(c) and 457(h) of the Securities Act of
             1933, as amended.
  
  
  <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 Registrant's International Employee Stock Purchase
  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 CERTAIN 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 filed 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 prospectus 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 the Registration Statement on
                             Form 8-A filed by the Registrant with the Commission
                             on March 2, 1999.
  
           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.
  
  Item 5. INTERESTS OF NAMED EXPERTS AND COUNSEL
  
           Not applicable.
  
  Item 6. INDEMNIFICATION OF DIRECTORS AND OFFICERS
  
           Section 67 of Chapter 156B of the Massachusetts General Laws provides
  that a corporation may indemnify its directors and officers to the extent
  specified in or authorized by (1) the articles of organization; (2) a by-law
  adopted by the stockholders; or (3) a vote adopted by the holders of a majority
  of the shares of stock entitled to vote on the election of directors. In all
  instances, the extent to which a corporation provides indemnification to its
  directors and officers under Section 67 is optional. 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
  
  
                                        -2-
  <PAGE>   3
  
  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 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 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.       The 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 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 Section 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 in 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
  
  
  
                                        -3-
  <PAGE>   4
  
  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.
  
  
  
  
  
  
  
                                        -4-
  <PAGE>   5
  
  
  
  
                                     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 Andover, Commonwealth of Massachusetts, on this 28th
  day of February, 2000.
  
                                               MKS INSTRUMENTS, INC.
  
  
                                               By: /s/ John R. Bertucci
                                                   -------------------------------
                                                   John R. Bertucci
                                                   Chairman of the Board and
                                                   Chief Executive Officer
  
                                  POWER OF ATTORNEY
  
  We, the undersigned officers and directors of MKS Instruments, Inc., hereby
  severally constitute John R. Bertucci, Ronald C. Weigner and Richard S. Chute,
  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 of 1933, this Registration
  Statement has been signed below by the following persons in the capacities and
  on the date indicated.
  
  <TABLE>
  <CAPTION>
             Signature                               Title                               Date
             ---------                               -----                               ----
  
  <S>                             <C>                                             <C>
  /s/ John R. Bertucci            Chairman of the Board and Chief                 February 28, 2000
  ------------------------------  Executive Officer (Principal Executive
  John R. Bertucci                Officer)
  
  /s/ Ronald C. Weigner           Vice President and Chief Financial              February 28, 2000
  ------------------------------  Officer (Principal Financial &
  Ronald C. Weigner               Accounting Officer)
  
  /s/ Richard S. Chute            Director                                        February 28, 2000
  ------------------------------
  Richard S. Chute
  
  /s/ Owen W. Robbins             Director                                        February 28, 2000
  ------------------------------
  Owen W. Robbins
  
  /s/ Robert J. Therrien          Director                                        February 28, 2000
  ------------------------------
  Robert J. Therrien
  
  /s/ Louis P. Valente            Director                                        February 28, 2000
  ------------------------------
  Louis P. Valente
  </TABLE>
  
  
  
  
                                        -5-
  <PAGE>   6
  
  
  
  
                                    EXHIBIT INDEX
  
  
  Exhibit
  Number                                   Description
  -------                                  -----------
  
  4.1(1)         Articles of Organization of the Registrant, as amended.
  
  4.2(1)         Bylaws of the Registrant, as amended.
  
  4.3(1)         Specimen Certificate for shares of Common Stock, no par value per
                 share, of the Registrant
  
  5              Opinion of Hale and Dorr LLP
  
  10.27          MKS Instruments, Inc. International Employee Stock Purchase Plan
  
  23.1           Consent of Hale and Dorr LLP (included in Exhibit 5)
  
  23.2           Consent of PricewaterhouseCoopers LLP
  
  24             Power of Attorney (included on the signature page of this
                 Registration Statement)
  
  
  
  
  
  
  
  
  -----------------
  (1)      Incorporated herein by reference to the Registrant's Registration
  Statement on Form S-1 (Commission File No. 333-71363).
  
  
  
  
  
                                        -6-
metadata_documents: [{"type": "S-8", "sequence": "1", "description": "MKS INSTRUMENTS, INC.", "secsgml_size_bytes": 16202}]
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": "333-31224", "film-number": "554674"}, "business-address": {"street1": "512 SHATTUCK RD", "city": "ANDOVER", "state": "MA", "zip": "01810", "phone": "9789752350"}, "mail-address": {"street1": "512 SHATTUCK ROAD", "city": "ANDOVER", "state": "MA", "zip": "01810"}}
text: 1
  
     As filed with the Securities and Exchange Commission on February 28, 2000
  
                                                        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
           (State or Other Jurisdiction of Incorporation or Organization)
  
                                     04-2277512
                        (I.R.S. Employer Identification No.)
  
  SIX SHATTUCK ROAD, ANDOVER, MASSACHUSETTS                        01810
   (Address of Principal Executive Offices)                      (Zip Code)
  
                         MKS INSTRUMENTS, INC. INTERNATIONAL
                            EMPLOYEE STOCK PURCHASE PLAN
                              (Full Title of the Plan)
  
                                  JOHN R. BERTUCCI
                                MKS INSTRUMENTS, INC.
                                  SIX SHATTUCK ROAD
                            ANDOVER, MASSACHUSETTS 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
        TITLE OF                                        MAXIMUM
       SECURITIES                                      OFFERING           PROPOSED MAXIMUM          AMOUNT OF
          TO BE                   AMOUNT               PRICE PER         AGGREGATE OFFERING       REGISTRATION
       REGISTERED            TO BE REGISTERED            SHARE                 PRICE                    FEE
  ------------------------------------------------------------------------------------------------------------
  Common Stock,
  no par value                   50,000                $43.56(1)           $2,178,000(1)             $574.99
  ------------------------------------------------------------------------------------------------------------
  (1)        Estimated solely for the purpose of calculating the registration fee,
             and based upon the average of the high and low prices of the Common
             Stock on the Nasdaq National Market on February 24, 2000 in
             accordance with Rules 457(c) and 457(h) of the Securities Act of
             1933, as amended.
  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 Registrant's International Employee Stock Purchase
  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 CERTAIN 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 filed 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 prospectus 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 the Registration Statement on
                             Form 8-A filed by the Registrant with the Commission
                             on March 2, 1999.
  
           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.
  
  Item 5. INTERESTS OF NAMED EXPERTS AND COUNSEL
  
           Not applicable.
  
  Item 6. INDEMNIFICATION OF DIRECTORS AND OFFICERS
  
           Section 67 of Chapter 156B of the Massachusetts General Laws provides
  that a corporation may indemnify its directors and officers to the extent
  specified in or authorized by (1) the articles of organization; (2) a by-law
  adopted by the stockholders; or (3) a vote adopted by the holders of a majority
  of the shares of stock entitled to vote on the election of directors. In all
  instances, the extent to which a corporation provides indemnification to its
  directors and officers under Section 67 is optional. 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
  
                                        -2-
  3
  
  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 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 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.       The 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 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 Section 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 in 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
  
                                        -3-
  4
  
  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.
  
                                        -4-
  5
  
                                     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 Andover, Commonwealth of Massachusetts, on this 28th
  day of February, 2000.
  
                                               MKS INSTRUMENTS, INC.
  
                                               By: /s/ John R. Bertucci
                                                   -------------------------------
                                                   John R. Bertucci
                                                   Chairman of the Board and
                                                   Chief Executive Officer
  
                                  POWER OF ATTORNEY
  
  We, the undersigned officers and directors of MKS Instruments, Inc., hereby
  severally constitute John R. Bertucci, Ronald C. Weigner and Richard S. Chute,
  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 of 1933, this Registration
  Statement has been signed below by the following persons in the capacities and
  on the date indicated.
  Signature                               Title                               Date
             ---------                               -----                               ----
  /s/ John R. Bertucci            Chairman of the Board and Chief                 February 28, 2000
  ------------------------------  Executive Officer (Principal Executive
  John R. Bertucci                Officer)
  
  /s/ Ronald C. Weigner           Vice President and Chief Financial              February 28, 2000
  ------------------------------  Officer (Principal Financial &
  Ronald C. Weigner               Accounting Officer)
  
  /s/ Richard S. Chute            Director                                        February 28, 2000
  ------------------------------
  Richard S. Chute
  
  /s/ Owen W. Robbins             Director                                        February 28, 2000
  ------------------------------
  Owen W. Robbins
  
  /s/ Robert J. Therrien          Director                                        February 28, 2000
  ------------------------------
  Robert J. Therrien
  
  /s/ Louis P. Valente            Director                                        February 28, 2000
  ------------------------------
  Louis P. Valente
  -5-
  6
  
                                    EXHIBIT INDEX
  
  Exhibit
  Number                                   Description
  -------                                  -----------
  
  4.1(1)         Articles of Organization of the Registrant, as amended.
  
  4.2(1)         Bylaws of the Registrant, as amended.
  
  4.3(1)         Specimen Certificate for shares of Common Stock, no par value per
                 share, of the Registrant
  
  5              Opinion of Hale and Dorr LLP
  
  10.27          MKS Instruments, Inc. International Employee Stock Purchase Plan
  
  23.1           Consent of Hale and Dorr LLP (included in Exhibit 5)
  
  23.2           Consent of PricewaterhouseCoopers LLP
  
  24             Power of Attorney (included on the signature page of this
                 Registration Statement)
  
  -----------------
  (1)      Incorporated herein by reference to the Registrant's Registration
  Statement on Form S-1 (Commission File No. 333-71363).
  
                                        -6-