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)
  
           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)
  
  
               AMENDED AND RESTATED 1999 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       250,000          $22.86            $5,715,000            $525.78
  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. 333-78069, relating to the Registrant's Amended and Restated
  1999 Employee Stock Purchase Plan, As Amended.
  <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
  --------------------                Executive Officer (Principal
  John R. Bertucci                    Executive Officer)
  
  
  /s/ Ronald C. Weigner               Vice President and Chief            June 14, 2002
  --------------------                Financial Officer (Principal
  Ronald C. Weigner                   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
  </TABLE>
  
  <PAGE>
  
  <TABLE>
  <S>                                 <C>                                 <C>
  /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
  
  <TABLE>
  <CAPTION>
   Exhibit
   Number                             Description
   ------                             -----------
  <S>        <C>
  4.1 (1)    Restated Articles of Organization of the Registrant.
  
  4.2 (2)    Articles of Amendment.
  
  4.3 (3)    Articles of Amendment
  
  4.4 (4)    Amended and Restated By-Laws of the Registrant.
  
  4.5 (4)    Specimen Certificate for Common Stock of the Registrant.
  
  5.1        Opinion of 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).
  </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 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": "b43366m3sv8.txt", "description": "MKS INSTRUMENTS, INC.", "secsgml_size_bytes": 7847}]
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": "333-90502", "film-number": "02679021"}, "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)
  
               AMENDED AND RESTATED 1999 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       250,000          $22.86            $5,715,000            $525.78
  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. 333-78069, relating to the Registrant's Amended and Restated
  1999 Employee Stock Purchase Plan, As Amended.
  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
  --------------------                Executive Officer (Principal
  John R. Bertucci                    Executive Officer)
  
  /s/ Ronald C. Weigner               Vice President and Chief            June 14, 2002
  --------------------                Financial Officer (Principal
  Ronald C. Weigner                   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.