<PAGE>
                                                                     EXHIBIT 4.6

                                SIPEX CORPORATION

                             STOCK OPTION AGREEMENT


         SIPEX Corporation (the "Company") hereby grants the following option to
purchase Common Stock, $.01 par value per share (the "Common Stock"), of the
Company pursuant to an action of the Board of Directors (the "Board") of the
Company on 5/17/01. The terms and conditions attached hereto are also a part
hereof.



<TABLE>
<S>                                                               <C>
         Name of employee (the "Employee" or "Optionee"):         EVA FERGUSON

         Date of this option grant:                               5/17/01

         Number of shares of the Company's Common Stock
         subject to this option ("Option Shares"):                5,000

         Option exercise price per share:                         $12.1400

         Number of Option Shares subject to vesting schedule:     5,000

         Vesting Start Date:                                      5/17/01
</TABLE>

                  Vesting Schedule. If the Employee has continued to be employed
         by the Company or any Related Corporation (as defined in Section 2
         hereof) on the following dates, the Employee may exercise this option
         for the number of shares of Common Stock as set forth below.


<TABLE>
<CAPTION>
<S>                                                        <C>
                  Prior to 5/17/02                         0 shares

                  As of 5/17/02 but prior to 5/17/03       1,000 shares

                  As of 5/17/03 but prior to 5/17/04       an additional 1,000 shares

                  As of 5/17/04 but prior to 5/17/05       an additional 1,000 shares

                  As of 5/17/05 but prior to 5/17/06       an additional 1,000 shares

                  As of 5/17/06                            an additional 1,000 shares

                  Payment alternatives:                    Section 6(a) (i) through (iv)
</TABLE>



                                                SIPEX CORPORATION

          /s/ Eva Ferguson
         -----------------------------------
         EVA FERGUSON  Signature of Optionee

         -----------------------------------
         36862 CHERRY STREET, APT. 136
                                                By: /s/ James E. Donegan
         -----------------------------------        ----------------------------
         NEWARK, CA   94560                         James E. Donegan
                                                    Chairman and Chief Executive
                                                     Officer

SIPEX CORPORATION
<PAGE>
STOCK OPTION AGREEMENT -- INCORPORATED TERMS AND CONDITIONS

         1.       GRANT AS NON-QUALIFIED STOCK OPTION. This option is a
non-statutory stock option and is not intended to qualify as an incentive stock
option under Section 422 of the Internal Revenue Code of 1986, as amended, and
the regulations thereunder (the "Code").

         2.       VESTING OF OPTION IF EMPLOYMENT CONTINUES. The Employee may
exercise this option for the number of shares of Common Stock set forth on the
vesting schedule on the cover page hereof if the Employee has continued to be
employed by the Company or any present or future parent or subsidiary of the
Company (collectively, "Related Corporations") in accordance with such vesting
schedule. Notwithstanding the foregoing, the Board may, in its discretion,
accelerate the date that any installment of this option becomes exercisable. The
foregoing rights are cumulative and (subject to Sections 3 or 4 hereof if the
Employee ceases to be employed by the Company) may be exercised only before the
date which is ten years from the date of this option grant.

         3.       TERMINATION OF EMPLOYMENT.

                  (a)      TERMINATION OTHER THAN FOR CAUSE. If the Employee
ceases to be employed by the Company and all Related Corporations, other than by
reason of death or disability as defined in Section 4 or termination for Cause
as defined in Section 3(c), no further installments of this option shall become
exercisable, and this option shall terminate (and may no longer be exercised)
after the passage of three months from the Employee's last day of employment,
but in no event later than the scheduled expiration date. In such a case, the
Employee's only rights hereunder shall be those which are properly exercised
before the termination of this option.

                  (b)      TERMINATION FOR CAUSE. If the employment of the
Employee is terminated for Cause (as defined in Section 4(c)), this option shall
terminate upon the Employee's receipt of written notice of such termination and
shall thereafter not be exercisable to any extent whatsoever.

                  (c)      DEFINITION OF CAUSE. "Cause" shall mean conduct
involving one or more of the following: (i) the substantial and continuing
failure of the Employee, after notice thereof, to render services to the Company
or any Related Entity in accordance with the terms or requirements of his
employment or business relationship; (ii) disloyalty, gross negligence, willful
misconduct, dishonesty, fraud or breach of fiduciary duty to the Company or any
Related Entity; (iii) the commission of an act of embezzlement or fraud; (iv)
deliberate disregard of the rules or policies of the Company or any Related
Entity, or breach of an employment or other agreement with the Company or any
Related Entity, which results in direct or indirect loss, damage or injury to
the Company or any Related Entity; (v) the unauthorized disclosure of any trade
secret or confidential information of the Company or any Related Entity; or (vi)
the commission of an act which constitutes unfair competition with the Company
or any Related Entity or which induces any customer or supplier to breach a
contract with the Company or any Related Entity.

         4.       DEATH; DISABILITY.

                  (a)      DEATH. If the Employee dies while in the employ of
the Company or any Related Corporation, this option may be exercised, to the
extent otherwise exercisable on the date of his or her death, by the Employee's
estate, personal representative or beneficiary to whom this option has been
assigned pursuant to Section 9, at any time within 180 days after the date of
death, but not later than the scheduled expiration date.

                  (b)      DISABILITY. If the Employee ceases to be employed by
the Company and all Related Corporations by reason of his disability, this
option may be exercised, to the extent otherwise exercisable on the date of the
termination of his employment, at any time within 180 days after such
termination, but not later than the scheduled expiration date. The term
disability means "permanent and total disability" as defined in Section 22(e)(3)
of the Code.
<PAGE>
                  (c)      EFFECT OF TERMINATION. At the expiration of the
180-day period provided in paragraph (a) or (b) of this Section 4 or the
scheduled expiration date, whichever is the earlier, this option shall terminate
(and shall no longer be exercisable) and the only rights hereunder shall be
those as to which the option was properly exercised before such termination.

         5.       PARTIAL EXERCISE. This option may be exercised in part at any
time and from time to time within the above limits, except that this option may
not be exercised for a fraction of a share.

         6.       PAYMENT OF ALTERNATIVES. (a) The exercise price shall be paid
in the following manner:

                           (i)      in cash or by check;

                           (ii)     subject to paragraph 6(b) below, by delivery
                                    of shares of the Company's Common Stock
                                    having a fair market value equal, as of the
                                    date of exercise, to the option exercise
                                    price;

                           (iii)    by delivery of an assignment satisfactory in
                                    form and substance to the Company of a
                                    sufficient amount of the proceeds from the
                                    sale of the Option Shares and an instruction
                                    to the broker or selling agent to pay that
                                    amount to the Company; or

                           (iv)     by any combination of the foregoing.

                  In the case of (ii) above, fair market value as of the date of
         exercise shall be determined as of the last business day for which such
         prices or quotes are available prior to the date of exercise and shall
         mean (i) the average (on that date) of the high and low prices of the
         Common Stock on the principal national securities exchange on which the
         Common Stock is traded, if the Common Stock is then traded on a
         national securities exchange; or (ii) the last reported sale price (on
         that date) of the Common Stock on the Nasdaq National Market, if the
         Common Stock is not then traded on a national securities exchange; or
         (iii) the closing bid price (or average of bid prices) last quoted (on
         that date) by an established quotation service for over-the-counter
         securities, if the Common Stock is not reported on the Nasdaq National
         Market. If the Common Stock is not publicly traded at the time of
         exercise, "fair market value" shall mean the fair value of the Common
         Stock as determined by the Board after taking into consideration all
         factors which it deems appropriate, including, without limitation,
         recent sale and offer prices of the Common Stock in private
         transactions negotiated at arm's length.

                  (b)      LIMITATIONS ON PAYMENT BY DELIVERY OF COMMON STOCK.
If the Employee delivers Common Stock held by the Employee ("Old Stock") to the
Company in full or partial payment of the option price, and the Old Stock so
delivered is subject to restrictions or limitations imposed by agreement between
the Employee and the Company, an equivalent number of Option Shares shall be
subject to all restrictions and limitations applicable to the Old Stock to the
extent that the Employee paid for the Option Shares by delivery of Old Stock, in
addition to any restrictions or limitations imposed by this Agreement.
Notwithstanding the foregoing, the Employee may not pay any part of the exercise
price hereof by transferring Common Stock to the Company unless such Common
Stock has been owned by the Employee free of any substantial risk of forfeiture
for at least six months.

         7.       METHOD OF EXERCISING OPTION. Subject to the terms and
conditions of this Agreement, this option may be exercised by written notice to
the Company at its principal executive office, or to such transfer agent as the
Company shall designate. Such notice shall state the election to exercise this
option and the number of Option Shares for which it is being exercised and shall
be signed by the person or persons so exercising this option. Such notice shall
be accompanied by payment of the full purchase price of such shares, and the
Company shall deliver a certificate or certificates representing such shares as
soon as practicable after the notice shall be received. Such certificate or
certificates shall be registered in the
<PAGE>
name of the person or persons so exercising this option (or, if this option
shall be exercised by the Employee and if the Employee shall so request in the
notice exercising this option, shall be registered in the name of the Employee
and another person jointly, with right of survivorship). In the event this
option shall be exercised, pursuant to Section 4 hereof, by any person or
persons other than the Employee, such notice shall be accompanied by appropriate
proof of the right of such person or persons to exercise this option.


         8.       SECURITIES LAWS RESTRICTIONS ON RESALE. Until registered under
the Securities Act of 1933, as amended, or any successor statute (the
"Securities Act"), the Option Shares will be of an illiquid nature and will be
deemed to be "restricted securities" for purposes of the Securities Act.
Accordingly, such shares must be sold in compliance with the registration
requirements of the Securities Act or an exemption therefrom. Unless the Option
Shares have been registered under the Securities Act, each certificate
evidencing any of the Option Shares shall bear a legend substantially as
follows:

         "The shares represented by this certificate are subject to restrictions
         on transfer and may not be sold, exchanged, transferred, pledged,
         hypothecated or otherwise disposed of except in accordance with and
         subject to all the terms and conditions of a certain Stock Option
         Agreement dated as of 5/17/01, a copy of which the Company will furnish
         to the holder of this certificate upon request and without charge."

         9.       OPTION NOT TRANSFERABLE. This option is not transferable or
assignable except by will or by the laws of descent and distribution. During the
Optionee's lifetime only the Optionee can exercise this option.

         10.      NO OBLIGATION TO EXERCISE OPTION. The grant and acceptance of
this option imposes no obligation on the Optionee to exercise it.

         11.      ADJUSTMENTS. In the event of any stock split, stock dividend,
extraordinary cash dividend, recapitalization, reorganization, merger,
consolidation, combination, exchange of shares, liquidation, spin-off, split-up,
or other similar change in capitalization or event, the number and class of
securities, vesting schedule and exercise price per share of this option shall
be adjusted by the Company (or a substituted option may be granted) to the
extent the Board shall determine, in good faith, that such an adjustment (or
substitution) is appropriate. Except as is expressly provided in this Section
11, no adjustment shall be made for dividends or similar rights for which the
record date is prior to such date of exercise.

         12.      WITHHOLDING TAXES. If the Company or any Related Corporation
in its discretion determines that it is obligated to withhold any tax in
connection with the exercise of this option, or in connection with the transfer
of, or the lapse of restrictions on, any Common Stock or other property acquired
pursuant to this option, the Optionee hereby agrees that the Company or any
Related Corporation may withhold from the Optionee's wages or any other
remuneration the appropriate amount of tax. At the discretion of the Company or
Related Corporation, the amount required to be withheld may be withheld in cash
from such wages or other remuneration or in kind from the Common Stock or other
property otherwise deliverable to the Optionee on exercise of this option. The
Optionee further agrees that, if the Company or Related Corporation does not
withhold an amount from the Optionee's wages or other remuneration sufficient to
satisfy the withholding obligation of the Company or Related Corporation, the
Optionee will make reimbursement on demand, in cash, for the amount
underwithheld.

         13.      ARBITRATION. Any dispute, controversy, or claim arising out
of, in connection with, or relating to the performance of this Agreement or its
termination shall be settled by arbitration in the Commonwealth of
Massachusetts, pursuant to the rules then obtaining of the American Arbitration
Association. Any award shall be final, binding and conclusive upon the parties
and a judgment rendered thereon may be entered in any court having jurisdiction
thereof.
<PAGE>
         14.      PROVISION OF DOCUMENTATION TO OPTIONEE. By signing this
Agreement the Optionee acknowledges receipt of a copy of this Agreement.

         15.      MISCELLANEOUS.

                  (a)      NOTICES. All notices hereunder shall be in writing
and shall be deemed given when sent by certified or registered mail, postage
prepaid, return receipt requested, if to the Optionee, to the address set forth
on the cover pages hereof or at the address shown on the records of the Company,
and if to the Company, to the Company's principal executive offices, attention
of the Corporate Secretary.

                  (b)      ENTIRE AGREEMENT; MODIFICATION. This Agreement
constitutes the entire agreement between the parties relative to the subject
matter hereof, and supersedes all proposals, written or oral, and all other
communications between the parties relating to the subject matter of this
Agreement. This Agreement may be modified, amended or rescinded only by a
written agreement executed by both parties.

                  (c)      FRACTIONAL SHARES. If this option becomes exercisable
for a fraction of a share because of the adjustment provisions contained herein,
such fraction shall be rounded down.

                  (d)      NO RIGHTS AS STOCKHOLDER. The Optionee shall not have
any rights as a stockholder with respect to any shares of Common Stock for which
this option is exercisable until such Optionee becomes the record holder of such
shares of Common Stock.

                  (e)      NO OBLIGATION OF CONTINUED EMPLOYMENT. This Agreement
imposes no obligation on the Company or Related Corporation to continue the
employment of the Employee. The Company expressly reserves the right any time to
dismiss or terminate its relationship with the Employee free from any liability
or claim under this Agreement.

                  (f)      ISSUANCES OF SECURITIES; CHANGES IN CAPITAL
STRUCTURE. Except as expressly provided herein, no issuance by the Company of
shares of stock of any class, or securities convertible into shares of stock of
any class, shall affect, and no adjustment by reason thereof shall be made with
respect to, the number or price of shares subject to this option. No adjustments
need be made for dividends paid in cash or in property other than securities of
the Company. If there shall be any change in the Common Stock of the Company
through merger, consolidation, reorganization, recapitalization, stock dividend,
stock split, combination or exchange of shares, spin-off, split-up or other
similar change in capitalization or event, the restrictions contained in this
Agreement shall apply with equal force to additional and/or substitute
securities, if any, received by the Optionee in exchange for, or by virtue of
his or her ownership of, Option Shares, except as otherwise determined by the
Board.

                  (g)      DISSOLUTION OR LIQUIDATION. In the event of the
proposed dissolution or liquidation of the Company, then the Board shall, as to
outstanding options, at its discretion provide, upon written notice to the
Optionee (i) that all options must be exercised, to the extent then exercisable,
within a specified number of days of the date of such notice, at the end of
which period, the options shall terminate or (ii) that such options (including
those which have not yet vested) shall be exercisable within a specified number
of days of such notice, at the end of which period the options shall terminate.

                  (h)      SEVERABILITY. The invalidity, illegality or
unenforceability of any provision of this Agreement shall in no way affect the
validity, legality or enforceability of any other provision.

                  (i)      SUCCESSORS AND ASSIGNS. This Agreement shall be
binding upon and inure to the benefit of the parties hereto and their respective
successors and assigns, subject to the limitations set forth in Section 9
hereof.
<PAGE>
                  (j)      GOVERNING LAW. This Agreement shall be governed by
and interpreted in accordance with the laws of the Commonwealth of
Massachusetts, without giving effect to the principles of the conflicts of laws
thereof.