Document:

<PAGE>

                           STOCK PURCHASE AGREEMENT
                           ------------------------

                                     Dated

                               December 20, 2000

                                    between

                         Berman, DeValerio & Pease, LLP
                             (the "Escrow Agent"),

                                      and

                         Richard Jones (the "Investor")

                                  INTRODUCTION

     WHEREAS, Pegasystems Inc. (the "Company") has issued shares of the
Company's Common Stock, $.01 par value per share, to the Escrow Agent to fund a
portion of a Settlement Fund that has been established pursuant to a Stipulation
of Compromise and Settlement entered into on September 22, 2000 between the
Company and the parties (the "Stipulation") in an action entitled Gelfer v.
Pegasystems Inc., Civil Action No. 98 Civ. 12527-JLT filed in federal court in
Massachusetts (the "Action").

     WHEREAS, the Escrow Agent wishes to sell, and the Investor wishes to
purchase from the Escrow Agent 115,000 shares of the Common Stock (the "Shares")
of the Company.

                              TERMS AND CONDITIONS

     ARTICLE I. REPRESENTATIONS AND WARRANTIES OF THE ESCROW AGENT

     The Escrow Agent represents and warrants to the Investor that:

     Section 1.01. Power and Authority Relative to this Transaction.  The Escrow
Agent possesses all requisite power and authority necessary to carry out the
transactions contemplated by this Agreement.  None of the actions taken by the
Escrow Agent pursuant to this Agreement will violate any provision of law, rule
or regulation or will result in the breach of or constitute a default under any
material agreement or instrument to which the Escrow Agent is a party or by
which it is bound, or result in the creation or imposition of any lien, charge,
restriction, claim or encumbrance of any nature whatsoever upon any of the
properties or assets of the Escrow Agent
<PAGE>

or upon the Shares. Each of this Agreement and all other documents contemplated
hereby, when executed and delivered by the Escrow Agent, will constitute valid
and legally binding obligations of the Escrow Agent, enforceable against the
Escrow Agent in accordance with their respective terms.

     Section 1.02.  Title to Shares.  As of immediately prior to the
consummation of the transactions contemplated hereby, all of the Shares will be
owned of record by the Escrow Agent, and the Escrow Agent will have good and
marketable title to the Shares, free and clear of all liens and encumbrances.
Immediately following the consummation of the transactions contemplated hereby,
the Investor will have good and marketable title to the Shares, free and clear
of all liens and encumbrances.

     ARTICLE II. REPRESENTATIONS AND WARRANTIES OF THE INVESTOR

     The Investor represents and warrants to the Escrow Agent that:

     Section 2.01. Power and Authority Relative to this Transaction. The
Investor has full power and authority and has taken all required action
necessary to permit him to execute and deliver and to carry out the terms of
this Agreement and all other documents or instruments required hereby.

     Section 2.02. Brokers, Etc. The Investor has dealt with no broker, finder,
commission agent or person in connection with the offer or sale of the Shares
and the transactions contemplated by this Agreement and is under no obligation
to pay any broker's fee, finder's fee, or commission in connection with such
transactions.

                          ARTICLE III. THE INVESTMENT

     Section 3.01. The Shares. Subject to the terms and conditions hereof, and
in reliance on the representations and warranties contained herein, the Escrow
Agent shall, subject to receipt of full payment therefor as contemplated by
Section 3.02, sell to the Investor, and the Investor shall purchase from the
Escrow Agent, the Shares at a purchase price equal to the average closing price
of the Shares, as reported on the Nasdaq National Market, for the ten trading
days immediately preceding the date of entry of the final court order approving
the Stipulation, which price equals $3.081 per Share (the "Purchase Price").

     Section 3.02. Consideration for the Shares. The Investor shall pay the
Purchase Price of the Shares purchased by him hereunder from the Escrow Agent in
full at the Closing referred to in Article IV by check or by wire transfer to
accounts designated in writing by the Escrow Agent.

                                       2
<PAGE>

                            ARTICLE IV. THE CLOSING

     The purchase and sale of the Shares being purchased by the Investor shall
take place at a closing (the "Closing") to be held at the offices of Choate,
Hall & Stewart, Boston, Massachusetts at 10:00 a.m. local time on the third
business day following the entry by the court of a judgment approving the
settlement of the Action as fair, reasonable and in the best interests of the
class, as provided in the Stipulation (or at such other place and time as may be
mutually agreed upon in writing).

                ARTICLE V. CONDITIONS OF THE CLOSING - INVESTOR

     The obligations of the Investor to purchase the Shares to be purchased by
him at the Closing shall be subject to the satisfaction of the following
conditions at and as of the time of the Closing:

     Section 5.01. Sale of Shares. The Escrow Agent shall have delivered to the
Investor the certificate[s] evidencing the Shares along with any documents
necessary to effect the transfer of the Shares.

              ARTICLE VI. CONDITIONS OF THE CLOSING - ESCROW AGENT

     The obligations of the Escrow Agent to sell the Shares to be purchased by
the Investor at the Closing shall be subject to the satisfaction of the
following conditions at and as of the time of the Closing:

     Section 6.01. Payment of Consideration. The Investor shall have paid the
Purchase Price of the Shares purchased by it hereunder.

                           ARTICLE VII. DEFINED TERMS

     The terms defined in this Article VII, whenever used and capitalized in
this Agreement, shall, unless the context otherwise requires, have the following
respective meanings:

     Closing:  shall have the meaning specified in Article IV.

     Company:  shall mean Pegasystems Inc., a Massachusetts corporation.

     Investor:  shall have the meaning specified at the beginning of this
Agreement.

     Shares:  shall have the meaning specified in the Introduction.

     1933 Act:  shall mean the Securities Act of 1933, as amended.

                                       3
<PAGE>

                          ARTICLE VIII. MISCELLANEOUS

     Section 8.01. Notices. All notices to a party hereunder shall be in writing
and shall be deemed to have been adequately given if delivered in person, upon
facsimile transmission with receipt acknowledged by the recipient's facsimile
transmission machine or by delivery by a recognized courier for overnight
delivery, or three days after having been mailed, certified mail, return receipt
requested, to such party at its address set forth below (or such other address
as it may from time to time designate in writing to the other parties hereto).

The Escrow Agent:  Berman, DeValerio & Pease, LLP
                   One Liberty Square
                   Boston, Massachusetts 02109

The Investor:      Richard Jones
                   c/o Pegasystems Inc.
                   101 Main Street
                   Cambridge, Massachusetts 02142

     Section 8.02. No Waiver. No failure to exercise and no delay in exercising,
on the part of the Investor, any right, power or privilege hereunder shall
operate as a waiver thereof; nor shall any single or partial exercise of any
right, power or privilege hereunder preclude any other or further exercise or
the exercise of any other right, power or privilege. The rights and remedies
herein provided are cumulative and not exclusive of any rights or remedies
provided by law.

     Section 8.03. Amendments and Waivers.  Except as hereinafter provided, this
Agreement may be modified or amended or any term or provision may be waived by a
writing signed by the Escrow Agent and the Investor. No waiver of any term or
provision shall be effective unless made in the same manner as an amendment of
such term or provision.

     Section 8.04.  Survival of Agreements etc.  All agreements, representations
and warranties contained herein or made in writing by or on behalf of the
Investor or the Escrow Agent in connection with the transactions contemplated
shall, except where otherwise provided by their terms, survive the execution and
delivery of this Agreement, the Closing pursuant to Article IV, and any
investigation at any time made by or on behalf of the Investor.

     Section 8.05. Construction. This Agreement shall be governed by and
construed in accordance with the law of The Commonwealth of Massachusetts. The
descriptive headings of the several Sections are for convenience only and shall
not control or affect the meaning or construction of any of the provisions.

                                       4
<PAGE>

     Section 8.06. Binding Effect and Benefits. This Agreement shall be binding
upon and shall inure to the benefit of the parties and their respective heirs,
successors and assigns, excluding assignees of the Shares, but not including any
purchaser thereof in whose hands the Shares are not a restricted security as
defined in Rule 144 under the 1933 Act.

     Section 8.07. Entire Agreement and Amendments. This Agreement constitutes
the entire agreement of the parties with respect to the subject matter and
neither this Agreement nor any provision may be waived, modified, amended or
terminated except by a written agreement signed by the parties, in accordance
with the provisions of Section 8.03. To the extent any term or other provision
of any other indenture, agreement or instrument by which any party is bound
conflicts with this Agreement, this Agreement shall have precedence over such
conflicting term or provision.

     Section 8.08. Counterparts. This Agreement may be executed in two or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

                                       5
<PAGE>

     IN WITNESS WHEREOF, the parties have executed this Agreement under seal as
of the date first above written.

Berman, DeValerio & Pease LLP
(as Escrow Agent)

By:   __________________________
Name: __________________________
Title:__________________________

Richard Jones:

________________________________

                                       6<PAGE>

                                                                   Exhibit 10.20

                           STOCK PURCHASE AGREEMENT
                           ------------------------

                                     Dated

                               December 20, 2000

                                    between

                         Berman, DeValerio & Pease, LLP
                             (the "Escrow Agent"),

                                      and

                       Edward B. Roberts (the "Investor")

                                  INTRODUCTION

     WHEREAS, Pegasystems Inc. (the "Company") has issued shares of the
Company's Common Stock, $.01 par value per share, to the Escrow Agent to fund a
portion of a Settlement Fund that has been established pursuant to a Stipulation
of Compromise and Settlement entered into on September 22, 2000 between the
Company and the parties (the "Stipulation") in an action entitled Gelfer v.
Pegasystems Inc., Civil Action No. 98 Civ. 12527-JLT filed in federal court in
Massachusetts (the "Action").

     WHEREAS, the Escrow Agent wishes to sell, and the Investor wishes to
purchase from the Escrow Agent 100,000 shares of the Common Stock (the "Shares")
of the Company.

                              TERMS AND CONDITIONS

     ARTICLE I. REPRESENTATIONS AND WARRANTIES OF THE ESCROW AGENT

     The Escrow Agent represents and warrants to the Investor that:

     Section 1.01. Power and Authority Relative to this Transaction.  The Escrow
Agent possesses all requisite power and authority necessary to carry out the
transactions contemplated by this Agreement.  None of the actions taken by the
Escrow Agent pursuant to this Agreement will violate any provision of law, rule
or regulation or will result in the breach of or constitute a default under any
material agreement or instrument to which the Escrow Agent is a party or by
which it is bound, or result in the creation or imposition of any lien, charge,
restriction, claim or encumbrance of any nature whatsoever upon any of the
properties or assets of the Escrow Agent
<PAGE>

or upon the Shares. Each of this Agreement and all other documents contemplated
hereby, when executed and delivered by the Escrow Agent, will constitute valid
and legally binding obligations of the Escrow Agent, enforceable against the
Escrow Agent in accordance with their respective terms.

     Section 1.02.  Title to Shares.  As of immediately prior to the
consummation of the transactions contemplated hereby, all of the Shares will be
owned of record by the Escrow Agent, and the Escrow Agent will have good and
marketable title to the Shares, free and clear of all liens and encumbrances.
Immediately following the consummation of the transactions contemplated hereby,
the Investor will have good and marketable title to the Shares, free and clear
of all liens and encumbrances.

     ARTICLE II. REPRESENTATIONS AND WARRANTIES OF THE INVESTOR

     The Investor represents and warrants to the Escrow Agent that:

     Section 2.01. Power and Authority Relative to this Transaction. The
Investor has full power and authority and has taken all required action
necessary to permit him to execute and deliver and to carry out the terms of
this Agreement and all other documents or instruments required hereby.

     Section 2.02. Brokers, Etc. The Investor has dealt with no broker, finder,
commission agent or person in connection with the offer or sale of the Shares
and the transactions contemplated by this Agreement and is under no obligation
to pay any broker's fee, finder's fee, or commission in connection with such
transactions.

                          ARTICLE III. THE INVESTMENT

     Section 3.01. The Shares. Subject to the terms and conditions hereof, and
in reliance on the representations and warranties contained herein, the Escrow
Agent shall, subject to receipt of full payment therefor as contemplated by
Section 3.02, sell to the Investor, and the Investor shall purchase from the
Escrow Agent, the Shares at a purchase price equal to the average closing price
of the Shares, as reported on the Nasdaq National Market, for the ten trading
days immediately preceding the date of entry of the final court order approving
the Stipulation, which price equals $3.081 per Share (the "Purchase Price").

     Section 3.02. Consideration for the Shares. The Investor shall pay the
Purchase Price of the Shares purchased by him hereunder from the Escrow Agent in
full at the Closing referred to in Article IV by check or by wire transfer to
accounts designated in writing by the Escrow Agent.

                                      -2-
<PAGE>

                            ARTICLE IV. THE CLOSING

     The purchase and sale of the Shares being purchased by the Investor shall
take place at a closing (the "Closing") to be held at the offices of Choate,
Hall & Stewart, Boston, Massachusetts at 10:00 a.m. local time on the third
business day following the entry by the court of a judgment approving the
settlement of the Action as fair, reasonable and in the best interests of the
class, as provided in the Stipulation (or at such other place and time as may be
mutually agreed upon in writing).

                ARTICLE V. CONDITIONS OF THE CLOSING - INVESTOR

     The obligations of the Investor to purchase the Shares to be purchased by
him at the Closing shall be subject to the satisfaction of the following
conditions at and as of the time of the Closing:

     Section 5.01. Sale of Shares. The Escrow Agent shall have delivered to the
Investor the certificate[s] evidencing the Shares along with any documents
necessary to effect the transfer of the Shares.

              ARTICLE VI. CONDITIONS OF THE CLOSING - ESCROW AGENT

     The obligations of the Escrow Agent to sell the Shares to be purchased by
the Investor at the Closing shall be subject to the satisfaction of the
following conditions at and as of the time of the Closing:

     Section 6.01. Payment of Consideration. The Investor shall have paid the
Purchase Price of the Shares purchased by it hereunder.

                           ARTICLE VII. DEFINED TERMS

     The terms defined in this Article VII, whenever used and capitalized in
this Agreement, shall, unless the context otherwise requires, have the following
respective meanings:

     Closing:  shall have the meaning specified in Article IV.

     Company:  shall mean Pegasystems Inc., a Massachusetts corporation.

     Investor:  shall have the meaning specified at the beginning of this
Agreement.

     Shares:  shall have the meaning specified in the Introduction.

     1933 Act:  shall mean the Securities Act of 1933, as amended.

                                      -3-
<PAGE>

                          ARTICLE VIII. MISCELLANEOUS

     Section 8.01. Notices. All notices to a party hereunder shall be in writing
and shall be deemed to have been adequately given if delivered in person, upon
facsimile transmission with receipt acknowledged by the recipient's facsimile
transmission machine or by delivery by a recognized courier for overnight
delivery, or three days after having been mailed, certified mail, return receipt
requested, to such party at its address set forth below (or such other address
as it may from time to time designate in writing to the other parties hereto).

The Escrow Agent:  Berman, DeValerio & Pease, LLP
                   One Liberty Square
                   Boston, Massachusetts 02109

The Investor:      Edward B. Roberts
                   c/o Pegasystems Inc.
                   101 Main Street
                   Cambridge, Massachusetts 02142

     Section 8.02. No Waiver. No failure to exercise and no delay in exercising,
on the part of the Investor, any right, power or privilege hereunder shall
operate as a waiver thereof; nor shall any single or partial exercise of any
right, power or privilege hereunder preclude any other or further exercise or
the exercise of any other right, power or privilege. The rights and remedies
herein provided are cumulative and not exclusive of any rights or remedies
provided by law.

     Section 8.03. Amendments and Waivers.  Except as hereinafter provided, this
Agreement may be modified or amended or any term or provision may be waived by a
writing signed by the Escrow Agent and the Investor. No waiver of any term or
provision shall be effective unless made in the same manner as an amendment of
such term or provision.

     Section 8.04.  Survival of Agreements etc.  All agreements, representations
and warranties contained herein or made in writing by or on behalf of the
Investor or the Escrow Agent in connection with the transactions contemplated
shall, except where otherwise provided by their terms, survive the execution and
delivery of this Agreement, the Closing pursuant to Article IV, and any
investigation at any time made by or on behalf of the Investor.

     Section 8.05. Construction. This Agreement shall be governed by and
construed in accordance with the law of The Commonwealth of Massachusetts. The
descriptive headings of the several Sections are for convenience only and shall
not control or affect the meaning or construction of any of the provisions.

                                      -4-
<PAGE>

     Section 8.06. Binding Effect and Benefits. This Agreement shall be binding
upon and shall inure to the benefit of the parties and their respective heirs,
successors and assigns, excluding assignees of the Shares, but not including any
purchaser thereof in whose hands the Shares are not a restricted security as
defined in Rule 144 under the 1933 Act.

     Section 8.07. Entire Agreement and Amendments. This Agreement constitutes
the entire agreement of the parties with respect to the subject matter and
neither this Agreement nor any provision may be waived, modified, amended or
terminated except by a written agreement signed by the parties, in accordance
with the provisions of Section 8.03. To the extent any term or other provision
of any other indenture, agreement or instrument by which any party is bound
conflicts with this Agreement, this Agreement shall have precedence over such
conflicting term or provision.

     Section 8.08. Counterparts. This Agreement may be executed in two or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

                                      -5-
<PAGE>

     IN WITNESS WHEREOF, the parties have executed this Agreement under seal as
of the date first above written.

Berman, DeValerio & Pease LLP
(as Escrow Agent)

By:   __________________________
Name: __________________________
Title:__________________________

Edward B. Roberts:

________________________________

                                      -6-

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