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                                                               EXHIBIT 3(a)

                  CERTIFICATE OF DESIGNATION, PREFERENCES AND
            RIGHTS OF SERIES A JUNIOR PARTICIPATING PREFERRED STOCK

                                       of

                               CABOT CORPORATION

             Pursuant to Section 151 of the General Corporation Law
                            of the State of Delaware

        We, Robert A. Charpie, President, and Walter F. Greeley, Secretary, of
Cabot Corporation, a corporation organized and existing under the General
Corporation Law of the State of Delaware, in accordance with the provisions
of Section 103 thereof, DO HEREBY CERTIFY:

        That pursuant to the authority conferred upon the Board of Directors by
the Restated Certificate of Incorporation, as amended, of the said Corporation,
the said Board of Directors on November 14, 1986, adopted the following
resolution creating a series of 800,000 shares of Preferred Stock designated as
Series A Junior Participating Preferred Stock:

        VOTED, that pursuant to the authority vested in the Board of Directors
of this Corporation in accordance with the provisions of its Restated
Certificate of Incorporation, as amended, a series of Preferred Stock of the
Corporation be and it hereby is created, and that the designation and amount
thereof and the voting powers, preferences and relative, participating,
optional and other special rights of the shares of such series, and the
qualifications, limitations or restrictions thereof are as follows:

        Section 1. DESIGNATION AND AMOUNT. The shares of such series shall be
designated as "Series A Junior Participating Preferred Stock" and the number of
shares constituting such series shall be 800,000.

        Section 2. DIVIDENDS AND DISTRIBUTIONS.

        (A) Subject to the prior and superior rights of the holders of any
shares of any series of Preferred Stock ranking prior and superior to the
shares of Series A Junior Participating Preferred Stock with respect to
dividends, the holders of shares of Series A Junior Participating Preferred
Stock shall be entitled to receive, when, as and if declared by the Board of
Directors out of
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funds legally available for the purpose, quarterly dividends payable in cash
on the eleventh day of January, March, June and September in each year (each
such date being referred to herein as a "Quarterly Dividend Payment Date"),
commencing on the first Quarterly Dividend Payment Date after the first
issuance of a share or fraction of a share of Series A Junior Participating
Preferred Stock, in an amount per share (rounded to the nearest cent) equal to
the greater of (a) $25 or (b) subject to the provision for adjustment
hereinafter set forth, 100 times the aggregate per share amount of all cash
dividends, and 100 times the aggregate per share amount (payable in kind)
of all non-cash dividends or other distributions other than a dividend
payable in shares of Common Stock or a subdivision of the outstanding shares of
Common Stock (by reclassification or otherwise), declared (but not withdrawn)
on the common stock, par value $1.00 per share, of the Corporation (the "Common
Stock") since the immediately preceding Quarterly Dividend Payment Date, or,
with respect to the first Quarterly Dividend Payment Date, since the first
issuance of any share or fraction of a share of Series A Junior Participating
Preferred Stock. In the event the Corporation shall at any time after November
14, 1986 (the "Rights Declaration Date") (i) declare any dividend on Common
Stock payable in shares of Common Stock, (ii) subdivide the outstanding Common
Stock, or (iii) combine the outstanding Common Stock into a smaller number of
shares, then in each such case the amount to which holders of shares of Series
A Junior Participating Preferred Stock were entitled immediately prior to
such event under clause (b) of the preceding sentence shall be adjusted by
multiplying such amount by a fraction the numerator of which is the number of
shares of Common Stock outstanding immediately after such event and the
denominator of which is the number of shares of Common Stock that were
outstanding immediately prior to such event.

        (B) The Corporation shall declare a dividend or distribution on the
Series A Junior Participating Preferred Stock as provided in paragraph (A)
above immediately after it declares a dividend or distribution on the Common
Stock (other than a dividend payable in shares of Common Stock); provided that,
in the event no dividend or distribution shall have been declared on the Common
Stock during the period between any Quarterly Dividend Payment Date and the
next subsequent Quarterly Dividend Payment Date, a dividend of $25 per share on
the Series A

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Junior Participating Preferred Stock shall nevertheless be payable on such
subsequent Quarterly Dividend Payment Date.

        (C) Dividends shall begin to accrue and be cumulative on outstanding
shares of Series A Junior Participating Preferred Stock from the Quarterly
Dividend Payment Date next preceding the date of issue of such shares of Series
A Junior Participating Preferred Stock, unless the date of issue of such shares
is prior to the record date for the first Quarterly Dividend Payment Date, in
which case dividends on such shares shall begin to accrue from the date of
issue of such shares, or unless the date of issue is a Quarterly Dividend
Payment Date or is a date after the record date for the determination of
holders of shares of Series A Junior Participating Preferred Stock entitled
to receive a quarterly dividend and before such Quarterly Dividend Payment
Date, in either of which events such dividends shall begin to accrue and be
cumulative from such Quarterly Dividend Payment Date. Accrued but unpaid
dividends shall not bear interest. Dividends paid on the shares of Series A
Junior Participating Preferred Stock in an amount less than the total amount of
such dividends at the time accrued and payable on such shares shall be
allocated pro rata on a share-by-share basis among all such shares at the time
outstanding. The Board of Directors may fix a record date for the determination
of holders of shares of Series A Junior Participating Preferred Stock entitled
to receive payment of a dividend or distribution declared thereon, which record
date shall be no more than 30 days prior to the date fixed for the payment
thereof.

        Section 3. VOTING RIGHTS. The holders of shares of Series A Junior
Participating Preferred Stock shall have the following voting rights:

        (A) Subject to the provision for adjustment hereinafter set forth, each
share of Series A Junior Participating Preferred Stock shall entitle the holder
thereof to 100 votes on all matters submitted to a vote of the stockholders of
the Corporation. In the event the Corporation shall at any time after the
Rights Declaration Date (i) declare any dividend on Common Stock payable in
shares of Common Stock, (ii) subdivide the outstanding Common Stock, or (iii)
combine the outstanding Common Stock into a smaller number of shares, then in
each such case the number of votes per share to which

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holders of shares of Series A Junior Participating Preferred Stock were
entitled immediately prior to such event shall be adjusted by multiplying such
number by a fraction the numerator of which is the number of shares of Common
Stock outstanding immediately after such event and the denominator of which is
the number of shares of Common Stock that were outstanding immediately prior to
such event.

        (B) Except as otherwise provided herein, by the Restated Certificate of
Incorporation, as amended, or by law, the holders of shares of Series A Junior
Participating Preferred Stock and the holders of shares of Common Stock
shall vote together as one class on all matters submitted to a vote of
stockholders of the Corporation.

        (C) (i) If at any time dividends on any Series A Junior Participating
Preferred Stock shall be in arrears in an amount equal to six (6) quarterly
dividends thereon, the occurrence of such contingency shall mark the beginning
of a period (herein called a "default period") which shall extend until such
time when all accrued and unpaid dividends for all previous quarterly dividend
periods and for the current quarterly dividend period on all shares of Series A
Junior Participating Preferred Stock then outstanding shall have been declared
and paid or set apart for payment. During each default period, all holders of
Preferred Stock (including holders of the Series A Junior Participating
Preferred Stock) with dividends in arrears in an amount equal to six (6)
quarterly dividends thereon, voting as a class, irrespective of series, shall
have the right to elect two (2) Directors.

        (ii) During any default period, such voting right of the holders of
Series A Junior Participating Preferred Stock may be exercised initially at a
special meeting called pursuant to subparagraph (iii) of this Section 3(C) or
at any annual meeting of stockholders, and thereafter at annual meetings of
stockholders, provided that neither such voting right nor the right of the
holders of any other series of Preferred Stock, if any, to increase, in certain
cases, the authorized number of Directors shall be exercised unless the holders
of one-third in number of shares of Preferred Stock outstanding shall be
present in person or by proxy. The absence of a quorum of the holders of Common
Stock shall not affect the exercise by the holders of Preferred Stock of such
voting right. At any meeting at which the holders of

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Preferred Stock shall exercise such voting right initially during an existing
default period, they shall have the right, voting as a class, to elect
Directors to fill such vacancies, if any, in the Board of Directors as may then
exist up to two (2) Directors or, if such right is exercised at an annual
meeting, to elect two (2) Directors. If the number which may be so elected at
any special meeting does not amount to the required number, the holders of
the Preferred Stock shall have the right to make such increase in the number of
Directors as shall be necessary to permit the election by them of the required
number. After the holders of the Preferred Stock shall have exercised their
right to elect Directors in any default period and during the continuance of
such period, the number of Directors shall not be increased or decreased
except by vote of the holders of Preferred Stock as herein provided or pursuant
to the rights of any equity securities ranking senior to or PARI PASSU with
the Series A Junior Participating Preferred Stock.

        (iii) Unless the holders of Preferred Stock shall, during an existing
default period, have previously exercised their right to elect Directors, the
Board of Directors may order, or any stockholder or stockholders owning in the
aggregate not less than ten percent (10%) of the total number of shares of
Preferred Stock outstanding, irrespective of series, may request, the calling of
a special meeting of the holders of Preferred Stock, which meeting shall
thereupon be called by the President, a Vice-President or the Secretary of the
Corporation. Notice of such meeting and of any annual meeting at which holders
of Preferred Stock are entitled to vote pursuant to this paragraph (C) (iii)
shall be given to each holder of record of Preferred Stock by mailing a copy of
such notice to him at his last address as the same appears on the books of the
Corporation. Such meeting shall be called for a time not earlier than 20 days
and not later than 60 days after such order or request or in default of the
calling of such meeting within 60 days after such order or request, such meeting
may be called on similar notice by any stockholder or stockholders owning in the
aggregate not less than ten percent (10%) of the total number of shares of
Preferred Stock outstanding. Notwithstanding the provisions of this paragraph
(C)(iii), no such special meeting shall be called during the period within 60
days immediately preceding the date fixed for the next annual meeting of the
stockholders.

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        (iv) In any default period, the holders of Common Stock, and other
classes of stock of the Corporation if applicable, shall continue to be
entitled to elect the whole number of Directors until the holders of Preferred
Stock shall have exercised their right to elect two (2) Directors voting as a
class, after the exercise of which right (x) the Directors so elected by the
holders of Preferred Stock shall continue in office until their successors
shall have been elected by such holders or until the expiration of the default
period, and (y) any vacancy in the Board of Directors may (except as provided
in paragraph (C)(ii) of this Section 3) be filled by vote of a majority of the
remaining Directors theretofore elected by the holders of the class of stock
which elected the Director whose office shall have become vacant. References in
this paragraph (C) to Directors elected by the holders of a particular class of
stock shall include Directors elected by such Directors to fill vacancies as
provided in clause (y) of the foregoing sentence.

        (v) Immediately upon the expiration of a default period, (x) the right
of the holders of Preferred Stock as a class to elect Directors shall cease,
(y) the term of any Directors elected by the holders of Preferred Stock as a
class shall terminate, and (z) the number of Directors shall be such number as
may be provided for in the certificate of incorporation or by-laws irrespective
of any increase made pursuant to the provisions of paragraph (C)(ii) of this
Section 3 (such number being subject, however, to change thereafter in any
manner provided by law or in the certificate of incorporation or by-laws). Any
vacancies in the Board of Directors effected by the provisions of clauses (y)
and (z) in the preceding sentence may be filled by a majority of the remaining
Directors.

        (D) Except as set forth herein, holders of Series A Junior
Participating Preferred Stock shall have no special voting rights and their
consent shall not be required (except to the extent they are entitled to vote
with holders of Common Stock as set forth herein) for taking any corporate
action.

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        Section 4. CERTAIN RESTRICTIONS.
                   --------------------

        (A) Whenever quarterly dividends or other dividends or distributions
payable on the Series A Junior Participating Preferred Stock as provided in
Section 2 are in arrears, thereafter and until all accrued and unpaid dividends
and distributions, whether or not declared, on shares of Series A Junior
Participating Preferred Stock outstanding shall have been paid in full, the
Corporation shall not

                               (i) declare or pay dividends on, make any other 
              distributions on, or redeem or purchase or otherwise acquire for 
              consideration any shares of stock ranking junior (either as to 
              dividends or upon liquidation, dissolution or winding up) to the 
              Series A Junior Participating Preferred Stock;

                               (ii) declare or pay dividends on or make any 
              other distributions on any shares of stock ranking on a parity
              (either as to dividends or upon liquidation, dissolution  or
              winding up) with the Series A Junior Participating Preferred
              Stock, except dividends paid ratably on the Series A Junior
              Participating Preferred Stock and all such parity stock on which
              dividends are payable or in arrears in proportion to the
              total amounts to which the holders of all such shares are then
              entitled;    
                                                                             
                               (iii) redeem or purchase or otherwise acquire 
              for consideration shares of any stock ranking on a parity (either
              as to dividends or upon liquidation, dissolution or winding up)
              with the Series A Junior Participating Preferred Stock,
              provided that the Corporation may at any time redeem, purchase
              or otherwise acquire shares of any such parity stock in
              exchange for shares of any stock of the Corporation ranking
              junior (either as to dividends or upon dissolution, liquidation
              or winding up) to the Series A Junior Participating Preferred
              Stock;

                               (iv) purchase or otherwise acquire for 
              consideration any shares of Series A Junior Participating 
              Preferred Stock, or any

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              shares of stock ranking on a parity with the Series A Junior
              Participating Preferred Stock, except in accordance with a
              purchase offer made in writing or by publication (as determined
              by the Board of Directors) to all holders of such shares upon
              such terms as the Board of Directors, after consideration of
              the respective annual dividend rates and other relative rights
              and preferences of the respective series and classes, shall
              determine in good faith will result in fair and equitable
              treatment among the respective series or classes.

        (B) The Corporation shall not permit any subsidiary of the
Corporation to purchase or otherwise acquire for consideration any shares of
stock of the Corporation unless the Corporation could, under paragraph (A) of
this Section 4, purchase or otherwise acquire such shares at such time and in
such manner.

        Section 5. REACQUIRED SHARES. Any shares of Series A Junior
Participating Preferred Stock purchased or otherwise acquired by the
Corporation in any manner whatsoever shall be retired and cancelled promptly
after the acquisition thereof. All such shares shall upon their cancellation
become authorized but unissued shares of Preferred Stock and may be reissued as
part of a new series of Preferred Stock to be created by resolution or
resolutions of the Board of Directors, subject to the conditions and
restrictions on issuance set forth herein.

        Section 6. LIQUIDATION, DISSOLUTION OR WINDING UP. (A) Upon any
liquidation (voluntary or otherwise), dissolution or winding up of the
Corporation, no distribution shall be made to the holders of shares of stock
ranking junior (either as to dividends or upon liquidation, dissolution or
winding up) to the Series A Junior Participating Preferred Stock unless, prior
thereto, the holders of shares of Series A Junior Participating Preferred
Stock shall have received $225 per share, plus an amount equal to accrued and
unpaid dividends and distributions thereon, whether or not declared, to the
date of such payment (the "Series A Liquidation Preference"). Following the
payment of the full amount of the Series A Liquidation Preference, no
additional distributions shall be made to the holders of shares of Series A
Junior Participating Preferred Stock unless, prior thereto, the holders of
shares of Common Stock shall have received an

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amount per share (the "Common Adjustment") equal to the quotient obtained by
dividing (i) the Series A Liquidation Preference by (ii) 100 (as appropriately
adjusted as set forth in subparagraph C below to reflect such events as stock
splits, stock dividends and recapitalizations with respect to the Common Stock)
(such number in clause (ii) immediately above being referred to as the
"Adjustment Number"). Following the payment of the full amount of the Series A
Liquidation Preference and the Common Adjustment in respect of all outstanding
shares of Series A Junior Participating Preferred Stock and Common Stock,
respectively, holders of Series A Junior Participating  Preferred Stock and
holders of shares of Common Stock shall receive their ratable and proportionate
share of the remaining assets to be distributed in the ratio of the Adjustment
Number to one (1) with respect to such Preferred Stock and Common Stock, on a
per share basis, respectively.

        (B) In the event, however, that there are not sufficient assets
available to permit payment in full of the Series A Liquidation Preference and
the liquidation preferences of all other series of preferred stock, if any,
which rank on a parity with the Series A Junior Participating Preferred Stock,
then such remaining assets shall be distributed ratably to the holders of such
parity shares in proportion to their respective liquidation preferences. In
the event, however, that there are not sufficient assets available to permit
payment in full of the Common Adjustment, then such remaining assets shall be
distributed ratably to the holders of Common Stock.

        (C) In the event the Corporation shall at any time after the Rights
Declaration Date (i) declare any dividend on Common Stock payable in shares of
Common Stock, (ii) subdivide the outstanding Common Stock, or (iii) combine the
outstanding Common Stock into a smaller number of shares, then in each such
case the Adjustment Number in effect immediately prior to such event shall be
adjusted by multiplying such Adjustment Number by a fraction the numerator of
which is the number of shares of Common Stock outstanding immediately after
such event and the denominator of which is the number of shares of Common
Stock that were outstanding immediately prior to such event.

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        Section 7. CONSOLIDATION, MERGER, ETC. In case the Corporation shall
enter into any consolidation, merger, combination or other transaction in which
the shares of Common Stock are exchanged for or changed into other stock or
securities, cash and/or any other property, then in any such case the shares
of Series A Junior Participating Preferred Stock shall at the same time be
similarly exchanged or changed in an amount per share (subject to the provision
for adjustment hereinafter set forth) equal to 100 times the aggregate amount
of stock, securities, cash and/or any other property (payable in kind), as the
case may be, into which or for which each share of Common Stock is changed or
exchanged. In the event the Corporation shall at any time after the Rights
Declaration Date (i) declare any dividend on Common Stock payable in shares of
Common Stock, (ii) subdivide the outstanding Common Stock, or (iii) combine the
outstanding Common Stock into a smaller number of shares, then in each such
case the amount set forth in the preceding sentence with respect to the
exchange or change of shares of Series A Junior Participating Preferred Stock
shall be adjusted by multiplying such amount by a fraction the numerator of
which is the number of shares of Common Stock outstanding immediately after
such event and the denominator of which is the number of shares of Common Stock
that were outstanding immediately prior to such event.

        Section 8. REDEMPTION. The outstanding shares of Series A Junior
Participating Preferred Stock may be redeemed at the option of the Board of
Directors as a whole, but not in part, at any time, or from time to time, at a
cash price per share equal to 105 percent of (i) the product of the Adjustment
Number times the Average Market Value (as such term is hereinafter defined)
of the Common Stock, plus (ii) all dividends which on the redemption date have
accrued on the shares to be redeemed and have not been paid, or declared and a
sum sufficient for the payment thereof set apart, without interest. The
"Average Market Value" is the average of the closing sale prices of the Common
Stock during the 30 day period immediately preceding the date before the
redemption date on the Composite Tape for New York Stock Exchange Listed
Stocks, or, if such stock is not quoted on the Composite Tape, on the New York
Stock Exchange, or, if such stock is not listed on such Exchange, on the
principal United States securities exchange registered under the Securities
Exchange Act of 1934, as amended, on which such

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stock is listed, or, if such stock is not listed on any such exchange, the
average of the closing sale prices with respect to a share of Common Stock
during such 30-day period, as quoted on the National Association of Securities
Dealers, Inc. Automated Quotations System or any system then in use, or if no
such quotations are available, the fair market value of the Common Stock as
determined by the Board of Directors in good faith.

        Section 9. RANKING. The Series A Junior Participating Preferred Stock
shall rank junior to all other series of the Corporation's Preferred Stock as
to the payment of dividends and the distribution of assets, unless the terms of
any such series shall provide otherwise.

        Section 10. AMENDMENT. The Restated Certificate of Incorporation, as
amended, of the Corporation shall not be further amended in any manner which
would materially alter or change the powers, preferences or special rights of
the Series A Junior Participating Preferred Stock so as to affect them
adversely without the affirmative vote of the holders of a majority or more of
the outstanding shares of Series A Junior Participating Preferred Stock, voting
separately as a class.

        Section 11. FRACTIONAL SHARES. Series A Junior Participating Preferred
Stock may be issued in fractions of a share which shall entitle the holder,
in proportion to such holders fractional shares, to exercise voting rights,
receive dividends, participate in distributions and to have the benefit of all
other rights of holders of Series A Junior Participating Preferred Stock.

        IN WITNESS WHEREOF, we have executed and subscribed this Certificate
and do affirm the foregoing as true under the penalties of perjury this 2nd day
of December   , 1986.

                                     /s/ Robert A. Charpie
                                     ----------------------
                                     President

Attest:

/s/ Walter F. Greeley
- ----------------------
Secretary


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