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, ANDOVER, MASSACHUSETTS                         01810
  (Address of Principal Executive Offices)                        (Zip Code)
  
  
                   AMENDED AND RESTATED 1995 STOCK INCENTIVE PLAN
                              (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           Per Share               Price                    Fee
  ====================================================================================================================
  
  <S>                             <C>                  <C>                  <C>                     <C>
  Common Stock no par value        6,000,000            $21.19(1)           $127,140,000(1)            $31,785
  per share                          shares
  --------------------------------------------------------------------------------------------------------------------
  </TABLE>
  
  (1)  Computed in accordance with Rules 457(c) and (h) under the Securities Act
       of 1933, as amended and based upon the average high and low prices of
       Common Stock on January 25, 2001.
  
  
  <PAGE>   2
  
  
                       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. 333-78071, relating to the Registrant's Amended and Restated 1995
  Stock Incentive Plan.
  
  
  <PAGE>   3
  
  
                                     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
  Ronald C. Weigner                               Financial Officer (Principal
                                                  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>   4
  
  
                                    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              Amended and Restated 1995 Stock Incentive Plan and First Amendment thereto.
  
   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.
metadata_documents: [{"type": "S-8", "sequence": "1", "filename": "b37996mks-8.txt", "description": "MKS INSTRUMENTS,INC.", "secsgml_size_bytes": 7815}]
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": "333-54490", "film-number": "1517219"}, "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, ANDOVER, MASSACHUSETTS                         01810
  (Address of Principal Executive Offices)                        (Zip Code)
  
                   AMENDED AND RESTATED 1995 STOCK INCENTIVE PLAN
                              (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           Per Share               Price                    Fee
  ====================================================================================================================
  Common Stock no par value        6,000,000            $21.19(1)           $127,140,000(1)            $31,785
  per share                          shares
  --------------------------------------------------------------------------------------------------------------------
  (1)  Computed in accordance with Rules 457(c) and (h) under the Securities Act
       of 1933, as amended and based upon the average high and low prices of
       Common Stock on January 25, 2001.
  2
  
                       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. 333-78071, relating to the Registrant's Amended and Restated 1995
  Stock Incentive Plan.
  3
  
                                     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
  Ronald C. Weigner                               Financial Officer (Principal
                                                  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
  4
  
                                    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              Amended and Restated 1995 Stock Incentive Plan and First Amendment thereto.
  
   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.