content: <PAGE>
        As filed with the Securities and Exchange Commission on June 14, 2002
  
                                                        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)
  
  <TABLE>
  <S>                                                       <C>
           MASSACHUSETTS                                          04-2277512
  (State or Other Jurisdiction of                              (I.R.S. Employer
  Incorporation or Organization)                            Identification Number)
  
  SIX SHATTUCK ROAD, ANDOVER, MASSACHUSETTS                         01810
  (Address of Principal Executive Offices)                        (Zip Code)
  </TABLE>
  
               INTERNATIONAL EMPLOYEE STOCK PURCHASE PLAN, AS AMENDED
                              (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 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              25,000                   $22.86                $571,500                $52.58
  par value per share
  </TABLE>
  
  (1) 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 June 12, 2002.
  <PAGE>
                       STATEMENT OF INCORPORATION BY REFERENCE
  
           Except as otherwise set forth below, this Registration Statement on
  Form S-8 incorporates by reference the contents of the Registration Statement on
  Form S-8, File No. 333-31224, relating to the Registrant's International
  Employee Stock Purchase Plan.
  <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 Andover, Massachusetts on June 14, 2002.
  
                              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     June 14, 2002
  ---------------------
  John R. Bertucci                                Executive Officer (Principal
                                                  Executive Officer)
  
   /s/ Ronald C. Weigner                          Vice President and Chief            June 14, 2002
  ----------------------
  Ronald C. Weigner                               Financial Officer (Principal
                                                  Financial Officer and Principal
                                                  Accounting Officer)
  
   /s/ Robert R. Anderson                         Director                            June 14, 2002
  -------------------------
  Robert R. Anderson
  
   /s/ James G. Berges                            Director                            June 14, 2002
  -------------------------
  James G. Berges
  
   /s/ Richard S. Chute                           Director                            June 14, 2002
  --------------------------
  Richard S. Chute
  </TABLE>
  <PAGE>
  <TABLE>
  <S>                                             <C>                                 <C>
   /s/ Hans-Jochen Kahl                           Director                            June 14, 2002
  -------------------------
  Hans-Jochen Kahl
  
   /s/ Owen W. Robbins                            Director                            June 14, 2002
  ------------------------
  Owen W. Robbins
  
   /s/ Louis P. Valente                           Director                            June 14, 2002
  --------------------------
  Louis P. Valente
  </TABLE>
  <PAGE>
                                    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 Hale and Dorr LLP.
  
  23.1       Consent of Hale and Dorr LLP (included in Exhibit 5.1).
  
  23.2       Consent of PricewaterhouseCoopers LLP.
  
  23.3       Consent of KPMG 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) originally filed with the
            Securities and Exchange Commission on December 13, 2000, as amended.
  
  (2)       Incorporated herein by reference from the Registrant's Quarterly
            Report on Form 10-Q originally filed with the Securities and Exchange
            Commission on August 14, 2001.
  
  (3)       Incorporated herein by reference from the Registrant's Definitive
            Proxy originally filed with the Securities and Exchange Commission on
            April 16, 2002.
  
  (4)       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.
metadata_documents: [{"type": "S-8", "sequence": "1", "filename": "b43364m1sv8.txt", "description": "MKS INSTRUMENTS, INC.", "secsgml_size_bytes": 8344}]
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": "333-90498", "film-number": "02679017"}, "business-address": {"street1": "SIX SHATTUCK RD", "city": "ANDOVER", "state": "MA", "zip": "01810", "phone": "9789752350"}, "mail-address": {"street1": "SIX SHATTUCK ROAD", "city": "ANDOVER", "state": "MA", "zip": "01810"}}
text: As filed with the Securities and Exchange Commission on June 14, 2002
  
                                                        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, ANDOVER, MASSACHUSETTS                         01810
  (Address of Principal Executive Offices)                        (Zip Code)
  INTERNATIONAL EMPLOYEE STOCK PURCHASE PLAN, AS AMENDED
                              (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 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              25,000                   $22.86                $571,500                $52.58
  par value per share
  (1) 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 June 12, 2002.
  STATEMENT OF INCORPORATION BY REFERENCE
  
           Except as otherwise set forth below, this Registration Statement on
  Form S-8 incorporates by reference the contents of the Registration Statement on
  Form S-8, File No. 333-31224, relating to the Registrant's International
  Employee Stock Purchase Plan.
  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 June 14, 2002.
  
                              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     June 14, 2002
  ---------------------
  John R. Bertucci                                Executive Officer (Principal
                                                  Executive Officer)
  
   /s/ Ronald C. Weigner                          Vice President and Chief            June 14, 2002
  ----------------------
  Ronald C. Weigner                               Financial Officer (Principal
                                                  Financial Officer and Principal
                                                  Accounting Officer)
  
   /s/ Robert R. Anderson                         Director                            June 14, 2002
  -------------------------
  Robert R. Anderson
  
   /s/ James G. Berges                            Director                            June 14, 2002
  -------------------------
  James G. Berges
  
   /s/ Richard S. Chute                           Director                            June 14, 2002
  --------------------------
  Richard S. Chute
  /s/ Hans-Jochen Kahl                           Director                            June 14, 2002
  -------------------------
  Hans-Jochen Kahl
  
   /s/ Owen W. Robbins                            Director                            June 14, 2002
  ------------------------
  Owen W. Robbins
  
   /s/ Louis P. Valente                           Director                            June 14, 2002
  --------------------------
  Louis P. Valente
  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 Hale and Dorr LLP.
  
  23.1       Consent of Hale and Dorr LLP (included in Exhibit 5.1).
  
  23.2       Consent of PricewaterhouseCoopers LLP.
  
  23.3       Consent of KPMG 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) originally filed with the
            Securities and Exchange Commission on December 13, 2000, as amended.
  
  (2)       Incorporated herein by reference from the Registrant's Quarterly
            Report on Form 10-Q originally filed with the Securities and Exchange
            Commission on August 14, 2001.
  
  (3)       Incorporated herein by reference from the Registrant's Definitive
            Proxy originally filed with the Securities and Exchange Commission on
            April 16, 2002.
  
  (4)       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.