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                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                    Form 8-A


                For Registration of Certain Classes of Securities
                     Pursuant to Section 12(b) or (g) of the
                         Securities Exchange Act of 1934




                              MKS Instruments, Inc.
- --------------------------------------------------------------------------------
             (Exact name of registrant as specified in its charter)

            Massachusetts                              04-2277512
- ----------------------------------------  --------------------------------------
      (State of incorporation                        (IRS Employer
         or organization)                          Identification No.)

    Six Shattuck Road, Andover, MA                       01810
- ----------------------------------------  --------------------------------------
(Address of principal executive offices)               (Zip Code)

                                                       
If this form relates to the registration of a              If this form relates to the registration of a
class of securities pursuant to Section 12(b)              class of securities pursuant to Section 12(g)
of the Exchange Act and is effective                       of the Exchange Act and is effective
pursuant to General Instruction A.(c),                     pursuant to General Instruction A.(d),
please check the following box.   [  ]                     please check the following box.   [x]



Securities Act registration statement file number to which this form 
relates: 333-71363
         ---------
      (If applicable)

Securities to be registered pursuant to Section 12(b) of the Act:

          Title of Each Class                 Name of Each Exchange on Which
          to be so Registered                 Each Class is to be Registered
          -------------------                 ------------------------------


            
- ----------------------------------------  --------------------------------------


Securities to be registered pursuant to Section 12(g) of the Act:

                      Common Stock, no par value per share
- --------------------------------------------------------------------------------
                                (Title of Class)




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Item 1:  DESCRIPTION OF REGISTRANT'S SECURITIES TO BE REGISTERED.

     The description under the heading "Description of Capital Stock" relating
to the Registrant's Common Stock, no par value per share, in the Prospectus
included in the Registrant's Registration Statement on Form S-1 filed with the
Securities and Exchange Commission (the "Registration Statement on Form S-1")
(File No. 333-71363) is incorporated herein by reference.


Item 2:  EXHIBITS.

     The following exhibits are filed herewith (or incorporated by reference as
indicated below):

     1.   Restated Articles of Organization of the Registrant, as amended,
          incorporated by reference to Exhibit 3.1 to the Company's Registration
          Statement on Form S-1.

     2.   Form of Amended and Restated Articles of Organization, incorporated by
          reference to Exhibit 3.2 to the Company's Registration Statement on
          Form S-1.

     3.   Bylaws of the Registrant, as amended, incorporated by reference to
          Exhibit 3.3 to the Company's Registration Statement on Form S-1.

     4.   Form of Amended and Restated Bylaws of the Registrant, incorporated by
          reference to Exhibit 3.4 to the Company's Registration Statement on
          Form S-1.




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                                    SIGNATURE


         Pursuant to the requirements of Section 12 of the Securities Exchange
Act of 1934, the Registrant has duly caused this registration statement to be
signed on its behalf by the undersigned, thereto duly authorized.



                                       MKS Instruments, Inc.



                                       By:  /s/ John R. Bertucci                
                                            ------------------------------------
                                                John R. Bertucci
                                                Chairman, President and
                                                Chief Executive Officer


March 2, 1999