Document:

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                                                                  Exhibit 10 (i)
                                                                  --------------

                FIRST AMENDMENT TO THE SHARE OPTION PLAN OF 1995
                ------------------------------------------------

     THIS FIRST AMENDMENT TO THE SHARE OPTION PLAN OF 1995 is hereby approved
and adopted by Sigma-Aldrich Corporation (the "Company") effective as of the
1/st/ day of January, 1999.

     WHEREAS, the Company established its Share Option Plan of 1995 (the "Plan")
to provide for the granting of options to purchase Common Stock of the Company
to certain key employees of the Company and its subsidiaries; and

     WHEREAS, the Plan established by the Company and approved by the
Shareholders of the Company contemplated that, as specified in Paragraph 6 of
the Plan, no option could be exercised after a date ten (10) years from the date
the option was granted; and

     WHEREAS, with respect to "non-qualified" options to be granted by the
Company to employees of the Company and its subsidiaries located outside the
United States, the Company desires to modify the ten (10) year limitation period
described above by extending this exercise period as provided in this First
Amendment.

     NOW, THEREFORE, the following amendments and modifications are hereby made
a part of the Plan:

     1.   The last sentence of the first paragraph of Paragraph 6 of the Plan is
hereby deleted, and the following sentence is inserted as the last sentence of
the first paragraph of Paragraph 8;

     No option shall be exercisable for less than ten (10) share lots nor in any
     amount after a date ten (10) years from the date it was granted; provided,
     however, notwithstanding the foregoing ten year limitation, the Committee
     shall be authorized and empowered hereunder to grant, in its discretion,
     non-qualified options to employees of the Company, or its present and
     future subsidiaries, whose location is outside of the United States with
     option terms specifying a period longer than ten (10) years during which
     the non-qualified options may be exercised but in no
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     event shall such extended period exceed seventeen (17) years from the date
     said options are granted.

     2.   The penultimate paragraph contained in Paragraph 8 of the Plan is
hereby deleted, and the following paragraph is inserted as the penultimate
paragraph of Paragraph 8:

               Except as may be otherwise specified by the Committee with
     respect to non-qualified options granted under Paragraph 6 of this Plan,
     nothing in this Paragraph 8 shall be construed to permit the exercise of an
     option more than ten (10) years after the date it was granted.

     IN WITNESS WHEREOF, the Company has executed this First Amendment to the
Share Option Plan of 1995 as of the day and year first above written.

                                   SIGMA-ALDRICH CORPORATION

                               By: /s/ David R. Harvey
                                   -------------------
                                   Title:  President<PAGE>

                                                                  Exhibit 10 (j)
                                                                  --------------

                            SECOND AMENDMENT TO THE
                            -----------------------
              SIGMA-ALDRICH CORPORATION SHARE OPTION PLAN OF 1995
              ---------------------------------------------------

     This Second Amendment to the Share Option Plan of 1995 is hereby approved
and adopted by Sigma-Aldrich Corporation (the "Company") effective as of the
November 1, 1999.

     WHEREAS, the Company established its Share Option Plan of 1995 (the "Plan")
to provide for the granting of options to purchase Common Stock of the Company
to certain key employees of the Company and its subsidiaries; and

     WHEREAS, the Company desires to clarify that the employees of a subsidiary
which is sold will be treated in the same manner as employees whose employment
is terminated pursuant to the sale of assets; and

     WHEREAS, the Plan provides that each option shall be exercisable in such
manner, at such time or times and subject to such conditions or limitations as
shall be fixed by the Compensation Committee of the Board (the "Committee"), in
its sole discretion at the time such option is granted; and

     WHEREAS, the Company desires to clarify that the Committee, in certain
circumstances, may amend the terms and conditions with respect to which an
option is exercisable after the time an option is granted; and

     WHEREAS, the Plan established by the Company and approved by the
shareholders of the Company contemplated that with certain exceptions no option
would be exercisable within twelve months of grant; and

     WHEREAS, the Company desires to create exceptions to such twelve month
rule.

     NOW, THEREFORE, the following amendments and modifications are hereby made
a part of the Plan:
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    1.    The first two sentences of paragraph 6 of the Plan are hereby deleted
          and the following three sentences are inserted in their place:

Except as provided in paragraphs 8(b)(ii) and 8(b)(v), no option granted under
          this Plan may be exercised prior to the expiration of twelve (12)
          months from the date it is granted. The Committee may specify a longer
          period of time during which an option may not be exercised at the time
          each option is granted. Any period of non-exercise (whether shortened
          as provided in paragraphs 8(b)(ii) and 8(b)(v) or lengthened as
          provided in the preceding sentence) is hereinafter referred to as the
          "Non-exercise Period."

    2.    A new sentence is added after the first sentence in paragraph 8(b)(i)
          of the Plan as follows:

If a subsidiary of the Company ceases to be a subsidiary of the Company, an
          optionee who is employed by such former subsidiary and is no longer
          employed by either the Company or any current subsidiary of the
          Company shall be deemed to have terminated employment with the Company
          and every subsidiary of the Company and such termination shall be
          deemed to have been made by the Company without cause.

    3.    A new paragraph 8(b)(v) is added immediately following paragraph

          8(b)(iv) as follows:

          (V)  Certain Corporate Transactions. If an optionee's employment is
               ------------------------------
               terminated or deemed terminated with the Company and every
               subsidiary of the Company solely as a results of:

               (1)  the sale of a subsidiary of the Company,

               (2)  the sale of the assets of a portion of the business of the
                    Company or a subsidiary of the Company, or

               (3)  a corporate reorganization or restructuring,

    the Committee may, in its sole and absolute discretion, allow the optionee
    to exercise options (i) which are not yet otherwise exercisable and (ii)
    which have not been granted at least twelve (12) months prior to the date of
    such transaction.

     IN WITNESS WHEREOF, the Company has executed this Second Amendment to the
Share Option Plan of 1995 as of the day and year first above written.

                                  SIGMA-ALDRICH CORPORATION

                              By: /s/ Michael R. Hogan
                                  --------------------
                                  Vice President<PAGE>

                                                                  Exhibit 10 (k)
                                                                  --------------

                THIRD AMENDMENT TO THE SHARE OPTION PLAN OF 1995
                ------------------------------------------------

1.   Sigma-Aldrich Corporation (the "Company") has at the date set out below
     adopted the following sub-plan (the "UK Sub-Plan") pursuant to Paragraph 2
     of the Sigma-Aldrich Corporation Share Option Plan of 1995 (the "Plan") so
     that the Plan may be administered and operated for the benefit of key
     employees of the Company and its subsidiaries who are resident or may be
     potentially resident in the United Kingdom. The UK Sub-Plan consists of two
     parts, the Approved Sub-Plan and the Non-Approved Sub-Plan.

                             The Approved Sub-Plan
                             ---------------------

2.   The following paragraphs 3 to 22 inclusive set out the terms of the
     Approved Sub-Plan which will constitute the part of the Plan approved by
     the Commissioners of the UK Inland Revenue (the "Commissioners") under
     Schedule 9 of the Income and Corporation Taxes Act 1988 ("Schedule 9").

3.   The provisions of the Approved Sub-Plan shall be the provisions set out in
     the Plan with and subject to the following modifications in paragraphs 4 to
     22 inclusive.

4.   Unless the context requires all expressions shall have the same meaning as
     in the Plan, provided that all words and terms not otherwise defined shall
     have the meaning attributed by Schedule 9 which for the purposes hereof
     (but for no other purpose) shall take precedence.  In addition any
     references to any statutory enactments shall be construed as a reference to
     that enactment as for the time being amended or reenacted.  Where the
     context so admits or requires words importing the singular shall include
     the plural and vice versa and words importing the masculine shall include
     the feminine.

5.   The Shares over which options may be granted shall form part of the
     ordinary share capital (as defined in Section 832 (1) Income and
     Corporation Taxes Act 1988 ("ICTA 1988")) of the Company and shall comply
     with the requirements of Paragraphs 10 to 14 inclusive of Schedule 9.

6.   The companies participating in the approved Sub-Plan shall be the Company
     and all companies controlled by the Company within the meaning of Section
     840 ICTA 1988 and which have been nominated by the Company to participate
     for the time being in the Approved Sub-Plan (the "Participating
     Companies").

7.   The Shares to be acquired on the exercise of an option shall:

     7.1  be fully paid up;
     7.2  not be redeemable; and
     7.3  not be subject to any restrictions other than restrictions which
          attach to all shares or stocks of the same class.
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     For the purpose of this paragraph the term "restrictions" shall include
     restrictions which are deemed to attach to the shares or stocks under any
     contract, agreement, arrangement or condition as referred to in Paragraph
     13 of Schedule 9.

8.   In addition to the provisions set out in Paragraph 4 of the Plan, an option
     shall only be granted under the approved Sub-Plan to employees (other than
     directors) or directors (other than those who normally work less than 25
     hours per week exclusive of meal breaks) of Participating Companies.

9.   A person shall not participate in the approved Sub-Plan, that is to say,
     obtain or exercise rights under it, if at that time he is precluded from
     participation by Paragraph 8 of Schedule 9.

10.  No option shall be granted where the option price is less than Market Value
     on the date of the grant.  For this purpose "Market Value" shall be the
     market value of a share as determined in accordance with the provisions of
     Part (VIII) of the Taxation of Chargeable Gains Act 1992 and agreed for the
     purposes of the approved Sub-Plan in advance with the Shares Valuation
     Division of the Inland Revenue.

11.  In all cases the option price shall be determined and stated at the date of
     grant and any other terms, conditions or limitations imposed on an option
     in accordance with Paragraph 6 of the Plan shall be objective.

12.  Options shall be personal to the optionee and shall not be transferable
     otherwise than on the death of the optionee.

13.  An optionee shall where the context permits include his legal personal
     representative.

14.  Options shall not be transferable by the laws of dissent or distribution,
     but the personal representative(s) of the optionee may exercise options
     held by him at the date of death, in accordance with and subject to the
     provisions of Paragraph 8 of the Plan, provided in all cases that the
     option is exercised within 12 months of the optionee's death and the said
     optionee would not have been precluded at the date of his death from
     exercising the options pursuant Paragraph 8 of Schedule 9.

15.  For the avoidance of doubt it is stated that the Company shall be the
     grantor as defined in Paragraph 1(1) of Schedule 9.

16.  No option shall be granted to an employee or director of a Participating
     Company if the grant of that option would cause the aggregate market value
     of shares (determined at the time prescribed by Paragraph 28 of Schedule 9
     and calculated in accordance with the provisions of Schedule 9) which he
     can acquire under the Approved Sub-Plan and any other scheme approved under
     Schedule 9 (excluding any Savings-Related Share Option Scheme) and
     established by the grantor or by any associated company (as defined in
     Section 187(2) ICTA 1988) of the grantor to exceed the limit prescribed by
     Paragraph 28 of Schedule 9.
<PAGE>

17.  To the extent that any grant of an option exceeds the limit prescribed in
     paragraph 16 it shall be deemed to comprise such whole number of Shares, as
     nearly as possible equals, but does not exceed, such limit.

18.  The payment of the option price shall only be paid for in cash, cheque,
     banker's draft or such similar method and Paragraph 9 of the Plan shall for
     the purposes of the Approved Sub-Plan be construed accordingly.

19.  Subject to any legal impediment beyond the reasonable control of the
     Company, Shares shall either be transferred or issued to the optionee to
     satisfy an option within 30 days of its exercise.

20.  If at a time when the UK Approved Sub-Plan is approved by the Commissioners
     under Schedule 9, no adjustment to an outstanding option shall have effect
     unless it is made pursuant to a transaction that constitutes a variation
     for the purposes of Paragraph 29 of Schedule 9 and until it is approved by
     the Commissioners and Paragraph 11 of the Plan shall be amended
     accordingly.

21.1 If any company "the Acquiring Company" claims control of the Company as a
     result of making a general offer

     21.1.1    to acquire the whole of the issued ordinary share capital of the
          Company which is made on a condition such that if it is satisfied the
          person making the offer will have control of the Company; or

     21.1.2    to acquire all the shares in the Company which are the same class
          as the shares subject to the option;

then the optionee may at any time within the appropriate period, by agreement
with the Company and the Acquiring Company, release his options under the
Approved Sub-Plan (the "Old Option") in consideration of the grant to him of
options which are equivalent to the Old Options but relate to shares in the
Acquiring Company ("the New Options").

21.2 For the purposes of this paragraph 21 "the appropriate period" shall mean
     the period of 6 months beginning with the time when the Acquiring Company
     making the offer has obtained control of the Company and any condition
     subject to which the offer is made is satisfied.

21.3 The New Option shall only be regarded as equivalent to the Old Option if
     the conditions specified in Paragraph 15(3) of Schedule 9 are satisfied.

21.4 For the purposes of any application of the provisions of this Approved Sub-
     Plan any New Options granted pursuant to this paragraph 21 shall be
     regarded as having been granted at a time when the corresponding Old Option
     was granted.

21.5 The New Option shall be exercisable in the same manner as the corresponding
     Old Option released and subject to the Rules of the Approved Sub-Plan as
     they have effect immediately before the release except that, with effect
     from a release, reference to the "Company" and to "Shares" throughout these
     Rules (except for Rules 6 and 15) shall, in relation to the New Option, be
     construed respectively as references to
<PAGE>

          the Acquiring Company (or as the case may be, any other company in
          respect of whose shares the New Option is granted) and to the shares
          comprised in the New Option.

22.  No amendment or modification, made pursuant to Paragraph 15 of the Plan, to
     the Approved Sub-Plan, (at a time when the Approved Sub-Plan is approved by
     the Commissioners under Schedule 9), shall take effect until approved by
     the Commissioners.

The Non-Approved Sub-Plan
-------------------------

23.  The following paragraphs 24, 25, 26 and 27 set out the terms of the Non-
     Approved Sub-Plan which will constitute part of the Plan but will not be
     approved by the Commissioners.

24.  The provisions of the Non-Approved Sub-Plan shall be the provisions set out
     in the Plan and paragraphs 6, 10, and 11 of the Approved Sub-Plan.

25.  In addition for the purpose of the Non-Approved Sub-Plan the words "and
     agreed for the purposes of the Approved Sub-Plan in advance with the Shares
     Valuation Division of the Inland Revenue" referred to in paragraph 10 of
     the Approved Sub-Plan shall be deleted.

26.  In the event that a Tax Liability arises in respect of the grant or
     exercise of an option, the option will not be deemed to be granted or may
     it be exercised unless:

26.1       the Relevant Payer is able to deduct an amount equal to the whole of
        the Tax Liability from the optionee's or potential optionee's net pay
        for the relevant period; or

26.2       the optionee or potential optionee has paid to the Relevant Payer an
        amount equal to the Tax Liability; or

26.3       the sum of the amount that the optionee or potential optionee has
        paid to the Relevant Payer in respect of the Relevant Payer's obligation
        to satisfy the Tax Liability and the total amount that the Relevant
        Payer is able to deduct from the optionee's or potential optionee's net
        pay for the relevant period is equal to or more than the Tax Liability;
        or

26.4       the optionee or potential optionee has given irrevocable instructions
        to the Company's brokers (or other person acceptable to the Committee)
        for the sale of sufficient Shares acquired on the exercise of the option
        to realize an amount equal to the Tax Liability and the payment of the
        Tax Liability to the Relevant payer; or

26.5       the Committee determines otherwise.

27. For the purposes of paragraph 26 and this paragraph 27:

27.1.  "Tax Liability" shall mean the amount of all taxes and/or employees'
       social security contributions and all other social taxes which a
       Participating Company, any company controlled by the Company with the
<PAGE>

       meaning of Section 840 ICTA 1988 or any other person (other than an
       optionee or potential optionee) would be required to account for or pay
       to the UK Inland Revenue or other tax authority in respect of an option;
       and

27.2.  "Relevant Payer" shall mean the person or persons who are liable to
       account for or pay any Tax Liability.

Adopted by the Board of Directors on November 9, 1999.

                                  SIGMA-ALDRICH CORPORATION

                              By: /s/ Michael R. Hogan
                                  --------------------
                                  Vice President

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