UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

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

                                   FORM 8-K/A


                                 CURRENT REPORT
     Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934


                Date of Report (date of earliest event reported):
                               September 30, 2004


                       Cabot Microelectronics Corporation
             -----------------------------------------------------
             (Exact name of registrant as specified in its charter)


              Delaware                     000-30205           36-4324765
  -------------------------------         ------------      -----------------
  (State or other jurisdiction of         (Commission         (IRS Employer
           incorporation)                 File Number)       Identification)


                    870 Commons Drive, Aurora, Illinois 60504
               ---------------------------------------------------
               (Address of principal executive offices) (Zip Code)

                                 (630) 375-6631
              ----------------------------------------------------
              (Registrant's telephone number, including area code)

                                 Not applicable
          -------------------------------------------------------------
          (Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to
simultaneously satisfy the filing obligation of the registrant under any of the
following provisions:

|_| Written communications pursuant to Rule 425 under the Securities Act (17CFR
    230.425)

|_| Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17
    CFR240.14a-12)

|_| Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange
    Act (17 CFR 240.14d-2(b))

|_| Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange
    Act (17 CFR 240.13e-4(c))





EXPLANATORY NOTE

Cabot Microelectronics Corporation files this first amendment to its Current
Report on Form 8-K, which it filed on September 30, 2004, to correct a
typographical error in the parenthetical at the end of the first sentence.


Item 7.01   Regulation FD Disclosure.

            To address certain issues arising pursuant to a new rule proposed by
the Financial Accounting Standards Board and as permitted by the Second Amended
and Restated Cabot Microelectronics Corporation 2000 Equity Incentive Plan (the
"Plan"), on September 27, 2004 the Compensation Committee of Cabot
Microelectronics Corporation's (the "Corporation's") Board of Directors
accelerated to September 1, 2005 the vesting of those stock options granted to
employees, officers and directors under the Plan prior to September 27, 2004
that have an option price equal to or greater than the fair market value of the
shares of the Corporation on September 27, 2004 ($34.65), through amendment made
and effective as of September 27, 2004 to the grant agreements for such stock
options. Approximately 1.3 million options with varying remaining vesting
schedules of fewer than three years as of September 1, 2005 are subject to the
acceleration provision and become exercisable as of such date as a result.

         This information is being furnished pursuant to Item 7.01 of Form 8-K
and shall not be deemed "filed" for purposes of Section 18 of the Securities and
Exchange Act of 1934, as amended (the "Exchange Act"), or incorporated by
reference in any filing under the Securities Act of 1933, as amended, or the
Exchange Act, except as shall be expressly set forth by specific reference in
such filing.



                                   SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this report to be signed on its behalf by the
undersigned hereunto duly authorized.


                                  CABOT MICROELECTRONICS CORPORATION


Date: September 30, 2004          By: /s/ WILLIAM S. JOHNSON
                                      ----------------------
                                      William S. Johnson
                                      Vice President and Chief Financial Officer
                                      [Principal Financial Officer]