<PAGE>

                                                                   Exhibit 10.18

                             PORTAL SOFTWARE, INC.
                2000 INTERNATIONAL EMPLOYEE STOCK PURCHASE PLAN
                ------------------------------------------------
                     (as amended through February 1, 2002)

     I.    PURPOSE OF THE PLAN

           This International Employee Stock Purchase Plan is intended to
promote the interests of Portal Software, Inc., a Delaware Corporation, by
providing eligible employees of the Corporation's Foreign Subsidiaries with the
opportunity to acquire a proprietary interest in the Corporation through the
purchase of shares of Common Stock at periodic intervals.

           Capitalized terms herein shall have the meanings assigned to such
terms in the attached Appendix.

           All share numbers in the document reflect the 3-for-1 split of the
Common Stock authorized by the Board in April 1999 and approved by the
shareholders in April 1999 and the 2-for-1 split of the Common Stock effected on
January 19, 2000.

     II.   ADMINISTRATION OF THE PLAN

           The Plan Administrator shall have full authority to interpret and
construe any provision of the Plan and to adopt such rules and regulations for
administering the Plan as it may deem necessary. Decisions of the Plan
Administrator shall be final and binding on all parties having an interest in
the Plan.

     III.  STOCK SUBJECT TO PLAN

           A. The stock purchasable under the Plan shall be shares of authorized
but unissued or reacquired Common Stock, including shares of Common Stock
purchased on the open market. The number of shares of Common Stock reserved for
issuance over the term of the Plan and the U.S. Plan shall be limited to
12,677,789 shares, less any shares issued under the U.S. Plan. Such reserve
consists of (i) 5,738,293 shares initially reserved for purchase as of the
Effective Time, plus (ii) an additional 3,426,888 share increase effected
February 2001 pursuant to Section III.B. below plus (iii) an additional
3,512,608 share increase effected February 2002 pursuant to Section III.B.
below.

           B. The number of shares of Common Stock available for issuance under
 the Plan and the U.S. Plan in the aggregate shall automatically increase on the
first trading day of February each year during the term of the Plan, beginning
February 1, 2001, by an amount equal to two percent (2%) of the total number of
shares of Common Stock outstanding on the last trading day of the immediately
preceding January, but in no event shall any such annual increase exceed
4,000,000 shares.

           C. Should any change be made to the Common Stock by reason of any
stock split, stock dividend, recapitalization, combination of shares, exchange
of shares or other change affecting the outstanding Common Stock as a class
without the Corporation's receipt of consideration, appropriate adjustments
shall be made to (i) the maximum number and class of securities issuable under
the Plan and the U.S. Plan, (ii) the maximum number and class of

<PAGE>

securities purchasable per Participant on any one Purchase Date, (iii) the
maximum number and class of securities purchasable by all Participants in the
aggregate on any one Purchase Date (iv) the number and class of securities and
the price per share in effect under each outstanding purchase right in order to
prevent the dilution or enlargement of benefits thereunder and (v) the maximum
number and/or class of securities by which the share reserve is to increase
automatically each calendar year pursuant to the provisions of Section III.B.

     IV.   OFFERING PERIODS

           A. Shares of Common Stock shall be offered for purchase under the
Plan through a series of successive offering periods until such time as (i) the
maximum number of shares of Common Stock available for issuance under the Plan
shall have been purchased or (ii) the Plan shall have been sooner terminated.

           B. Each offering period shall be of such duration (not to exceed
twenty-four (24) months) as determined by the Plan Administrator prior to the
start date of that offering period. However, the initial offering period shall
commence at the Effective Time and terminate on the last business day in May
2001. The next offering period shall commence on the first business day in June
2001, and subsequent offering periods shall commence as designated by the Plan
Administrator.

           C. Each offering period shall be comprised of a series of one or more
successive Purchase Intervals. Purchase Intervals shall run from the first
business day in June to the last business day in November each year and from the
first business day in December each year to the last business day in May in the
following year.

           D. Should the Fair Market Value per share of Common Stock on any
Purchase Date within an offering period be less than the Fair Market Value per
share of Common Stock on the start date of that offering period, then that
offering period shall automatically terminate immediately after the purchase of
shares of Common Stock on such Purchase Date, and a new offering period shall
commence on the next business day following such Purchase Date. The new offering
period shall have a duration of twenty (24) months, unless a shorter duration is
established by the Plan Administrator within five (5) business days following
the start date of that offering period.

     V.    ELIGIBILITY

           A. Each individual who is an Eligible Employee on the start date of
any offering period under the Plan may enter that offering period on such start
date or on any subsequent Semi-Annual Entry Date within that offering period,
provided he or she remains an Eligible Employee.

           B. Each individual who first becomes an Eligible Employee after the
start date of an offering period may enter that offering period on any
subsequent Semi-Annual Entry Date within that offering period on which he or she
is an Eligible Employee.

           C. The date an individual enters an offering period shall be
designated his or her Entry Date for purposes of that offering period.


                                       2.

<PAGE>

           D. To participate in the Plan for a particular offering period, the
Eligible Employee must complete the enrollment forms prescribed by the Plan
Administrator (including a stock purchase agreement and a payroll deduction
authorization) and file such forms with the Plan Administrator (or its
designate) on or before his or her scheduled Entry Date.

     VI.   PAYROLL DEDUCTIONS

           A. Except to the extent otherwise provided in the Plan (or any
addendum thereto) or authorized by the Plan Administrator, the purchase price
for the shares of Common Stock acquired under the Plan shall be paid from
accumulated payroll deductions authorized by the Participant.

           B. The payroll deduction authorized by the Participant for purposes
of acquiring shares of Common Stock during an offering period may be any
multiple of one percent (1%) of the Cash Earnings paid to the Participant during
each Purchase Interval within that offering period, up to a maximum of fifteen
percent (15%). The deduction rate so authorized shall continue in effect
throughout the offering period, except to the extent such rate is changed in
accordance with the following guidelines:

                    (i)    The Participant may, at any time during the offering
           period, reduce his or her rate of payroll deduction to become
           effective as soon as possible after filing the appropriate form with
           the Plan Administrator. The Participant may not, however, effect more
           than one (1) such reduction per Purchase Interval.

                    (ii)   The Participant may, prior to the commencement of any
           new Purchase Interval within the offering period, increase the rate
           of his or her payroll deduction by filing the appropriate form with
           the Plan Administrator. The new rate (which may not exceed the
           fifteen percent (15%) maximum) shall become effective on the start
           date of the first Purchase Interval following the filing of such
           form.

           C. The payroll deduction authorized by the Participant shall be
collected in the currency in which paid by the Foreign Subsidiary. The payroll
deductions collected during each Purchase Interval shall be converted into U.S.
Dollars on the Purchase Date for that Purchase Interval on the basis of the
exchange rate in effect on the day before such Purchase Date. The Plan
Administrator shall have the absolute discretion to determine the applicable
exchange rate to be in effect for each Purchase Date by any reasonable method
which may be based on the exchange rate actually available in the ordinary
course of business on such date. Any changes or fluctuations in the exchange
rate at which the payroll deductions collected on the Participant's behalf are
converted into U.S. Dollars on each Purchase Date shall be borne solely by the
Participant.


                                       3.

<PAGE>

           D. Payroll deductions shall begin on the first pay day
administratively feasible following the Participant's Entry Date into the
offering period and shall (unless sooner terminated by the Participant) continue
through the pay day ending with or immediately prior to the last day of that
offering period. The amounts so collected shall be credited to the Participant's
book account under the Plan, initially in the currency in which paid by the
Foreign Subsidiary until converted into U.S. dollars on the Purchase Date.
Except to the extent otherwise provided by the Plan (including any addendum
thereto) or by the Plan Administrator, (i) no interest shall be paid on the
balance from time to time outstanding in such account, and (ii) the amounts
collected from the Participant shall not be required to be held in any
segregated account or trust fund and may be commingled with the general assets
of the Corporation and used for general corporate purposes.

           E. Payroll deductions shall automatically cease upon the termination
of the Participant's purchase right in accordance with the provisions of the
Plan.

           F. The Participant's acquisition of Common Stock under the Plan on
any Purchase Date shall neither limit nor require the Participant's acquisition
of Common Stock on any subsequent Purchase Date, whether within the same or a
different offering period.

     VII.  PURCHASE RIGHTS

           A. Grant of Purchase Right. A Participant shall be granted a separate
              -----------------------
purchase right for each offering period in which he or she participates. The
purchase right shall be granted on the Participant's Entry Date into the
offering period and shall provide the Participant with the right to purchase
shares of Common Stock, in a series of successive installments over the
remainder of such offering period, upon the terms set forth below. The
Participant shall execute a stock purchase agreement embodying such terms and
such other provisions (not inconsistent with the Plan) as the Plan Administrator
may deem advisable.

           Under no circumstances shall purchase rights be granted under the
Plan to any Eligible Employee if such individual would, immediately after the
grant, own (within the meaning of Code Section 424(d)) or hold outstanding
options or other rights to purchase, stock possessing five percent (5%) or more
of the total combined voting power or value of all classes of stock of the
Corporation or any Corporate Affiliate.

           B. Exercise of the Purchase Right. Each purchase right shall be
              ------------------------------
automatically exercised in installments on each successive Purchase Date within
the offering period, and shares of Common Stock shall accordingly be purchased
on behalf of each Participant on each such Purchase Date. The purchase shall be
effected by applying the Participant's payroll deductions for the Purchase
Interval ending on such Purchase Date, as converted into U.S. Dollars, to the
purchase of whole shares of Common Stock at the U.S. Dollar purchase price in
effect for the Participant for that Purchase Date.


                                       4.

<PAGE>

           C. Purchase Price. The U.S. Dollar purchase price per share at which
              --------------
Common Stock will be purchased on the Participant's behalf on each Purchase Date
within the offering period shall be equal to eighty-five percent (85%) of the
lower of (i) the Fair Market Value per share of Common Stock on the
-----
Participant's Entry Date into that offering period or (ii) the Fair Market Value
per share of Common Stock on that Purchase Date.

           D. Number of Purchasable Shares. The number of shares of Common Stock
              ----------------------------
purchasable by a Participant on each Purchase Date during the offering period
shall be the number of whole shares obtained by dividing the amount collected
from the Participant through payroll deductions during the Purchase Interval
ending with that Purchase Date, as converted into U.S. Dollars, by the U.S.
Dollar purchase price in effect for the Participant for that Purchase Date.
However, the maximum number of shares of Common Stock purchasable per
Participant on any one Purchase Date shall not exceed three thousand five
hundred (3,500) shares, subject to periodic adjustments in the event of certain
changes in the Corporation's capitalization. In addition, the maximum aggregate
number of shares of Common Stock purchasable by all Participants on any one
Purchase Date under the Plan and the U.S. Plan shall not exceed one million two
hundred thousand (1,200,000) shares, subject to periodic adjustments in the
event of certain changes in the Corporation's capitalization. However, the Plan
Administrator shall have the discretionary authority, exercisable prior to the
start of any offering period under the Plan, to increase or decrease the
limitations to be in effect for the number of shares purchasable per Participant
and in the aggregate by all Participants on each Purchase Date during that
offering period.

           E. Excess Payroll Deductions. Any payroll deductions not applied to
              -------------------------
the purchase of shares of Common Stock on any Purchase Date because they are not
sufficient to purchase a whole share of Common Stock shall be held for the
purchase of Common Stock on the next Purchase Date. However, any payroll
deductions not applied to the purchase of Common Stock by reason of the
limitation on the maximum number of shares purchasable per Participant or in the
aggregate on the Purchase Date shall be promptly refunded in the currency in
which collected by the Foreign Subsidiary.

           F. Termination of Purchase Right. The following provisions shall
              -----------------------------
govern the termination of outstanding purchase rights:

                    (i)    A Participant may, at any time prior to the date 5
           days preceding the next scheduled Purchase Date in the offering
           period, terminate his or her outstanding purchase right by filing the
           appropriate form with the Plan Administrator (or its designate), and
           no further payroll deductions shall be collected from the Participant
           with respect to the terminated purchase right. Any payroll deductions
           collected during the Purchase Interval in which such termination
           occurs shall, at the Participant's election, be immediately refunded
           in the currency in which collected by the Foreign Subsidiary or held
           for the purchase of shares on the next Purchase Date. If no such
           election is made at the time such purchase right is terminated, then
           the payroll deductions collected with respect to


                                       5.

<PAGE>

           the terminated right shall be refunded as soon as possible in the
           currency in which collected by the Foreign Subsidiary.

                    (ii)   The termination of such purchase right shall be
           irrevocable, and the Participant may not resume participation in the
           offering period for which the terminated purchase right was granted
           unless he or she re-enrolls in the Plan (by making a timely filing of
           the prescribed enrollment forms) on or before a regularly-scheduled
           Entry Date into that offering period. In such event, the Participant
           shall be granted a new purchase right with a new purchase price based
           upon the Fair Market Value per share on Common Stock on his or her
           new Entry Date.

                    (iii)  Should the Participant cease to remain an Eligible
           Employee for any reason (including death, disability or change in
           status) while his or her purchase right remains outstanding, then
           that purchase right shall immediately terminate, and all of the
           Participant's payroll deductions for the Purchase Interval in which
           the purchase right so terminates shall be immediately refunded in the
           currency in which collected by the Foreign Subsidiary. However,
           should the Participant cease to remain in active service by reason of
           an approved unpaid leave of absence, then the Participant shall have
           the right, exercisable up until the last business day of the Purchase
           Interval in which such leave commences, to (a) withdraw all the
           payroll deductions collected to date on his or her behalf for that
           Purchase Interval or (b) have such funds held for the purchase of
           shares on his or her behalf on the next scheduled Purchase Date. In
           no event, however, shall any further payroll deductions be collected
           on the Participant's behalf during such leave. Upon the Participant's
           return to active service (x) within ninety (90) days following the
           commencement of such leave or (y) prior to the expiration of any
           longer period for which such Participant's right to reemployment with
           the Corporation is guaranteed by either statute or contract, his or
           her payroll deductions under the Plan shall automatically resume at
           the rate in effect at the time the leave began, unless the
           Participant withdraws from the Plan prior to his or her return. An
           individual who returns to active employment following a leave of
           absence which exceeds in duration the applicable (x) or (y) time
           period will be treated as a new Employee for purposes of subsequent
           participation in the Plan and must accordingly re-enroll in the Plan
           (by making a timely filing of the prescribed enrollment forms) on or
           before his or her scheduled Entry Date into the offering period.

           G. Transfer of Employment. In the event that a Participant who is an
              ----------------------
Employee of a Foreign Subsidiary is transferred and becomes an Employee of the
Corporation during a Purchase Interval under the Plan, such individual shall
continue to remain a Participant in the Plan and payroll deductions shall
continue to be collected until the next Purchase Date as if the Participant had
remained an Employee of the Foreign Subsidiary.

           In the event that an Employee of the Corporation who is a participant
in the U.S. Plan is transferred and becomes an Employee of a Foreign Subsidiary
during a Purchase Interval in effect under the U.S. Plan, such individual shall
automatically become a Participant under the


                                       6.

<PAGE>

Plan for the duration of the offering period in effect at that time under the
Plan and the balance in such individual's book account maintained under the U.S.
Plan shall be transferred as a balance to a book account opened for such
individual under the Plan. Such balance, together with all other payroll
deductions collected from such individual by the Foreign Subsidiary for the
remainder of the Purchase Interval under the Plan (as converted into U.S.
Dollars), shall be applied on the next Purchase Date to the purchase of shares
under the Plan.

           H. Change in Control. Each outstanding purchase right shall
              -----------------
automatically be exercised, immediately prior to the effective date of any
Change in Control, by applying the payroll deductions of each Participant for
the Purchase Interval in which such Change in Control occurs to the purchase of
whole shares of Common Stock at a purchase price per share equal to eighty-five
percent (85%) of the lower of (i) the Fair Market Value per share of Common
Stock on the Participant's Entry Date into the offering period in which such
Change in Control occurs or (ii) the Fair Market Value per share of Common Stock
immediately prior to the effective date of such Change in Control. However, the
applicable limitation on the number of shares of Common Stock purchasable per
Participant shall continue to apply to any such purchase, but not the limitation
applicable to the maximum number of shares of Common Stock purchasable in the
aggregate. Payroll deductions not yet converted into U.S. Dollars at the time of
the Change in Control shall be converted from the currency in which paid by the
Foreign Subsidiary into U.S. Dollars on the basis of the exchange rate in effect
at as determined by the Plan Administrator at the time of the Change in Control.

           The Corporation shall use its best efforts to provide at least ten
(10)-days prior written notice of the occurrence of any Change in Control, and
Participants shall, following the receipt of such notice, have the right to
terminate their outstanding purchase rights prior to the effective date of the
Change in Control.

           I. Proration of Purchase Rights. Should the total number of shares of
              ----------------------------
Common Stock to be purchased pursuant to outstanding purchase rights on any
particular date exceed the number of shares then available for issuance under
the Plan and the U.S. Plan, the Plan Administrator shall make a pro-rata
allocation of the available shares on a uniform and nondiscriminatory basis, and
the payroll deductions of each Participant and each participant in the U.S.
Plan, to the extent in excess of the aggregate purchase price payable for the
Common Stock pro-rated to such individual, shall be refunded.

           J. Assignability. The purchase right shall be exercisable only by the
              -------------
Participant and shall not be assignable or transferable by the Participant.

           K. Stockholder Rights. A Participant shall have no stockholder rights
              ------------------
with respect to the shares subject to his or her outstanding purchase right
until the shares are purchased on the Participant's behalf in accordance with
the provisions of the Plan and the Participant has become a holder of record of
the purchased shares.

     VIII. ACCRUAL LIMITATIONS

           A. No Participant shall be entitled to accrue rights to acquire
Common Stock pursuant to any purchase right outstanding under this Plan if and
to the extent such accrual, when


                                       7.

<PAGE>

aggregated with (i) rights to purchase Common Stock accrued under any other
purchase right granted under this Plan and (ii) similar rights accrued under
other International Employee Stock Purchase Plans (within the meaning of Code
Section 423) of the Corporation or any Corporate Affiliate, would otherwise
permit such Participant to purchase more than Twenty-Five Thousand U.S. Dollars
(U.S. $25,000.00) worth of stock of the Corporation or any Corporate Affiliate
(determined on the basis of the Fair Market Value per share on the date or dates
such rights are granted) for each calendar year such rights are at any time
outstanding.

           B. For purposes of applying such accrual limitations to the purchase
rights granted under the Plan, the following provisions shall be in effect:

                    (i)    The right to acquire Common Stock under each
           outstanding purchase right shall accrue in a series of installments
           on each successive Purchase Date during the offering period on which
           such right remains outstanding.

                    (ii)   No right to acquire Common Stock under any
           outstanding purchase right shall accrue to the extent the Participant
           has already accrued in the same calendar year the right to acquire
           Common Stock under one (1) or more other purchase rights at a rate
           equal to Twenty-Five Thousand U.S. Dollars (U.S. $25,000.00) worth of
           Common Stock (determined on the basis of the Fair Market Value per
           share on the date or dates of grant) for each calendar year such
           rights were at any time outstanding.

           C. If by reason of such accrual limitations, any purchase right of a
Participant does not accrue for a particular Purchase Interval, then the payroll
deductions which the Participant made during that Purchase Interval with respect
to such purchase right shall be promptly refunded.

           D. In the event there is any conflict between the provisions of this
Article and one or more provisions of the Plan or any instrument issued
thereunder, the provisions of this Article shall be controlling.

     IX.   EFFECTIVE DATE AND TERM OF THE PLAN

           A. The Plan was adopted by the Board on June 13, 2000 and shall
become effective at the Effective Time, provided no purchase rights granted
                                        --------
under the Plan shall be exercised, and no shares of Common Stock shall be issued
hereunder until the Corporation shall have complied with all applicable
requirements of the 1933 Act (including the registration of the shares of Common
Stock issuable under the Plan on a Form S-8 registration statement filed with
the Securities and Exchange Commission), all applicable listing requirements of
any stock exchange (or the Nasdaq National Market, if applicable) on which the
Common Stock is listed for trading and all other applicable requirements
established by law or regulation.

           B. Unless sooner terminated by the Board, the Plan shall terminate
upon the earliest of (i) the last business day in May, 2009, (ii) the date on
         --------
which all shares available for issuance under the Plan shall have been sold
pursuant to purchase rights exercised under the Plan or (iii) the date on which
all purchase rights are exercised in connection with a Corporate


                                       8.

<PAGE>

Transaction. No further purchase rights shall be granted or exercised, and no
further payroll deductions shall be collected, under the Plan following such
termination.

     X.    AMENDMENT OF THE PLAN

           A. The Board may alter, amend, suspend or terminate the Plan at any
time to become effective immediately following the close of any Purchase
Interval. However, the Plan may be amended or terminated immediately upon Board
action, if and to the extent necessary to assure that the Corporation will not
recognize, for financial reporting purposes, any compensation expense in
connection with the shares of Common Stock offered for purchase under the Plan,
should the financial accounting rules applicable to the Plan at the Effective
Time be subsequently revised so as to require the recognition of compensation
expense in the absence of such amendment or termination.

           B. In no event may the Board effect any of the following amendments
or revisions to the Plan without the approval of the Corporation's stockholders:
(i) increase the number of shares of Common Stock issuable under the Plan,
except for permissible adjustments in the event of certain changes in the
Corporation's capitalization, (ii) alter the purchase price formula so as to
reduce the purchase price payable for the shares of Common Stock purchasable
under the Plan or (iii) modify the eligibility requirements for participation in
the Plan.

     XI.   GENERAL PROVISIONS

           A. All costs and expenses incurred in the administration of the Plan
shall be paid by the Corporation; however, each Plan Participant shall bear all
costs and expenses incurred by such individual in the sale or other disposition
of any shares purchased under the Plan.

           B. Nothing in the Plan shall confer upon the Participant any right to
continue in the employ of the Corporation or any Corporate Affiliate for any
period of specific duration or interfere with or otherwise restrict in any way
the rights of the Corporation (or any Corporate Affiliate employing such person)
or of the Participant, which rights are hereby expressly reserved by each, to
terminate such person's employment at any time for any reason, with or without
cause.

           C. Except to the extent otherwise provided in any addendum to the
Plan, the provisions of the Plan shall be governed by the laws of the State of
California without resort to that State's conflict-of-laws rules.

           D. A Foreign Subsidiary or the Corporation, as the case may be, shall
have the right to deduct from any payment to be made under this Plan, or to
otherwise require, prior to the issuance or delivery of any shares of Common
Stock or the payment of any cash, payment by each Participant, of any tax
required by applicable law to be withheld.

           E. Additional provisions for individual Foreign Subsidiaries may be
incorporated in one or more Addenda to the Plan. Such Addenda shall have full
force and effect with respect to the Foreign Subsidiaries to which they apply.
In the event of a conflict between


                                       9.

<PAGE>

the provisions of such an Addendum and one or more other provisions of the Plan,
the provisions of the Addendum shall be controlling.


                                       10.

<PAGE>

                                   Schedule A
                                   ----------

                         Corporations Participating in
                   International Employee Stock Purchase Plan

Portal Software Argentina S.R.L.
Portal Software International Pty Limited
Portal Software (Europe) Limited
Portal Software Canada Inc.
Portal Software France
Portal Software Germany GmbH
Portal Software (Hamburg) GmbH
42 Consulting-Group GmbH
Portal Software (Asia Pacific) Limited
Portal Software Italia S.r.l.
Portal Software Japan K.K.
Portal Software Mexico, S. de R.L. de C.V.
Portal Software Netherlands B.V.
Portal Software Korea Limited Company
Portal Software Informatica S.L.
Portal Software Sweden AB
Solution 42 Iberica S.L
Portal Software International Sarl
Portal Software Taiwan Limited

<PAGE>

                                    APPENDIX
                                    --------

           The following definitions shall be in effect under the Plan:

           A. Board shall mean the Corporation's Board of Directors.
              -----

           B. Cash Earnings shall mean the (i) regular base salary paid to a
              -------------
Participant by one or more Foreign Subsidiaries during such individual's period
of participation in one or more offering periods under the Plan plus (ii) all
overtime payments, bonuses, profit-sharing distributions and other
incentive-type payments received during such period. Such Cash Earnings shall be
calculated before deduction of (A) any income or employment tax withholdings or
(B) any and all contributions made by the Participant to any salary deferral
plan now or hereafter established by the Corporation or any Corporate Affiliate.
However, Cash Earnings shall not include any contributions made on the
Participant's behalf by the Corporation or any Corporate Affiliate to any
employee benefit or welfare plan now or hereafter established.

           C. Change in Control shall mean a change in ownership of the
              -----------------
Corporation pursuant to any of the following transactions:

                    (i)    a merger or consolidation in which securities
           possessing more than fifty percent (50%) of the total combined voting
           power of the Corporation's outstanding securities are transferred to
           a person or persons different from the persons holding those
           securities immediately prior to such transaction, or

                    (ii)   the sale, transfer or other disposition of all or
           substantially all of the assets of the Corporation in complete
           liquidation or dissolution of the Corporation, or

                    (iii)  the acquisition, directly or indirectly by an person
           or related group of persons (other than the Corporation or a person
           that directly or indirectly controls, is controlled by or is under
           common control with the Corporation) of beneficial ownership (within
           the meaning of Rule 13d-3 of the 1934 Act) of securities possessing
           more than fifty percent (50%) of the total combined voting power of
           the Corporation's outstanding securities pursuant to a tender or
           exchange offer made directly to the Corporation's stockholders.

           D. Code shall mean the U.S. Internal Revenue Code of 1986, as
              ----
amended.

           E. Common Stock shall mean the Corporation's common stock.
              ------------


                                      A-1.

<PAGE>

           F. Corporate Affiliate shall mean any parent or subsidiary
              -------------------
corporation of the Corporation (as determined in accordance with Code Section
424), whether now existing or subsequently established.

           G. Corporation shall mean Portal Software, Inc., a Delaware
              -----------
corporation, and any corporate successor to all or substantially all of the
assets or voting stock of Portal Software, Inc. which shall by appropriate
action adopt the Plan.

           H. Effective Time shall mean the time at which a new offering period
              --------------
after May 31, 2001 begins under the U.S. Plan. For example, if a new offering
period began on June 1, 2001, the Effective Time would be June 1, 2001.

           I. Eligible Employee shall mean any person who is employed by a
              -----------------
Foreign Subsidiary on a basis under which he or she is regularly expected to
render more than twenty (20) hours of service per week for more than five (5)
months per calendar year for earnings considered wages under Code Section
3401(a).

           J. Entry Date shall mean the date an Eligible Employee first
              ----------
commences participation in the offering period in effect under the Plan. The
earliest Entry Date under the Plan shall be the Effective Time.

           K. Fair Market Value per share of Common Stock on any relevant date
              -----------------
shall be determined in accordance with the following provisions:

                    (i)    If the Common Stock is at the time traded on the
           Nasdaq National Market, then the Fair Market Value shall be the U.S.
           Dollar closing selling price per share of Common Stock on the date in
           question, as such price is reported by the National Association of
           Securities Dealers on the Nasdaq National Market. If there is no
           closing selling price for the Common Stock on the date in question,
           then the Fair Market Value shall be the U.S. Dollar closing selling
           price on the last preceding date for which such quotation exists.

                    (ii)   If the Common Stock is at the time listed on any
           Stock Exchange, then the Fair Market Value shall be the U.S. Dollar
           closing selling price per share of Common Stock on the date in
           question on the Stock Exchange determined by the Plan Administrator
           to be the primary market for the Common Stock, as such price is
           officially quoted in the composite tape of transactions on such
           exchange. If there is no U.S. Dollar closing selling price for the
           Common Stock on the date in question, then the Fair Market Value
           shall be the U.S. Dollar closing selling price on the last preceding
           date for which such quotation exists.

           L. Foreign Subsidiary shall mean any non-U.S. Corporate Affiliate or
              ------------------
Affiliates as may be authorized from time to time by the Board to extend the
benefits of the Plan to their Eligible Employees. The Foreign Subsidiaries
participating in the Plan as of the Effective Date are listed in attached
Schedule A.

           M. 1933 Act shall mean the Securities Act of 1933, as amended.
              --------


                                      A-2.

<PAGE>

           N. Participant shall mean any Eligible Employee of a Participating
              -----------
Corporation who is actively participating in the Plan.

           O. Plan shall mean the Corporation's 2000 International Employee
              ----
Stock Purchase Plan, as set forth in this document.

           P. Plan Administrator shall mean the committee of two (2) or more
              ------------------
Board members appointed by the Board to administer the Plan.

           Q. Purchase Date shall mean the last U.S. business day of each
              -------------
Purchase Interval. The initial Purchase Date shall be six months after the
Effective Time.

           R. Purchase Interval shall mean each successive six (6)-month period
              -----------------
within the offering period at the end of which there shall be purchased shares
of Common Stock on behalf of each Participant.

           S. Semi-Annual Entry Date shall mean the first U.S. business day in
              ----------------------
June and December each year on which an Eligible Employee may first enter an
offering period.

           T. Stock Exchange shall mean either the American Stock Exchange or
              --------------
the New York Stock Exchange.

           U. Underwriting Agreement shall mean the agreement between the
              ----------------------
Corporation and the underwriter or underwriters managing the initial public
offering of the Common Stock.

           V. U.S. Plan shall mean the Portal Software, Inc. 1999 Employee Stock
              ---------
Purchase Plan.


                                      A-3.