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                                                                     EXHIBIT 4.1

Amendment dated September 29, 2000 to Rights Agreement.


                       FIRST AMENDMENT TO RIGHTS AGREEMENT

         AMENDMENT (this "Amendment"), dated as of September 29, 2000, between
Cabot Microelectronics Corporation, a Delaware corporation (the "Corporation"),
and EquiServe Trust Company, N.A., a national banking association (the "Rights
Agent").

                                   WITNESSETH:

         WHEREAS, the Corporation and the Rights Agent entered into the Rights
Agreement, dated as of March 24, 2000 (the "Rights Agreement");

         WHEREAS, pursuant to Section 27 of the Rights Agreement, the
Corporation may from time to time supplement or amend the Rights Agreement in
accordance with the provisions of such Section, among other things, to cure any
ambiguity; and

         WHEREAS, the Board of Directors of the Corporation has determined that
it is in the best interests of the Corporation and its shareholders that the
Rights Agreement be amended as provided herein.

         NOW THEREFORE, in consideration of the premises and the mutual
agreements herein set forth, the parties hereby agree as follows:

         Section 1. Definitions. Capitalized terms used herein without
definition shall have the meanings specified in the Rights Agreement.


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         Section 2. Amendment. The definition of "Grandfathered Person" set
forth in Section 1(j)(i) of the Rights Agreement shall be amended by deleting
clause (i) thereof and inserting a new clause (i) which shall read in its
entirety as follows:

                    (i) Cabot Corporation and any Subsidiary of Cabot
                Corporation until such time as Cabot Corporation and all
                Subsidiaries of Cabot Corporation, considered in the aggregate,
                shall cease to own at least 50% of the total outstanding equity
                securities of the Corporation, at which time Cabot Corporation
                and all Subsidiaries of Cabot Corporation shall permanently
                cease to constitute Grandfathered Persons;

         Section 3. Governing Law. This Amendment shall be deemed to be a
contract made under the laws of the State of Delaware and for all purposes shall
be governed by and construed in accordance with the laws of such State
applicable to contracts to be made and performed entirely within such State.

         Section 4. Counterparts. This Amendment may be executed in any number
of counterparts and each of such counterparts shall for all purposes be deemed
to be an original, and all such counterparts shall together constitute but one
and the same instrument.

         Section 5. Ratification. Except as expressly provided herein, this
Amendment shall not by implication or otherwise alter, modify, amend or in any
way affect any of the terms, conditions, obligations, covenants or agreements
contained in the Rights Agreement, all of which are ratified and confirmed in
all respects and shall continue in full force and effect.

         IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be
duly executed and attested, all as of the date and year first above written.


Attest:                                    CABOT MICROELECTRONICS CORPORATION


By: /s/ Catherine D. Mezo           By: /s/ H. Carol Bernstein
    ---------------------               ----------------------
    Name: Catherine D. Mezo             Name: H. Carol Bernstein
    Title: Executive Assistant          Title: VP, Secretary and General Counsel


Attest:                                    EQUISERVE TRUST COMPANY, N.A.


By: /s/ Dawn Brandmark              By: /s/ Carol Mulvey Eori
    ------------------                  ---------------------
    Name: Dawn Brandmark                Name: Carol Mulvey Eori
    Title: Account Manager              Title: Managing Director




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