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                                                                   EXHIBIT 10.31

                             TENNECO PACKAGING INC.
                                1900 FIELD COURT
                           LAKE FOREST, ILLINOIS 60045



                                 April 12, 1999


PCA Holdings, LLC
c/o Madison Dearborn Partners, Inc.
Three First National Plaza
Suite 3800
Chicago, IL 60602
Attn:    Samuel M. Mencoff
         Justin S. Huscher

                  Re:      Contribution Agreement

Gentlemen:

                  Reference is made to that certain Contribution Agreement,
dated as of January 25, 1999 (the "Contribution Agreement"), among Tenneco
Packaging Inc. ("TPI"), PCA Holdings LLC ("PCA"), and Packaging Corporation of
America ("Newco"). Capitalized terms used in this letter agreement not defined
herein shall have the meanings set forth in the Contribution Agreement.

                  The purpose of this letter agreement is to correct certain
errors or ambiguities that were included in the Contribution Agreement, and to
reflect the parties' agreements with respect to certain other matters, to the
extent those agreements differ form the terms of the Contribution Agreement

                  1.  The definition of "Assumed Indebtedness" in the 
         Contribution Agreement refers to the definition set forth in the
         Preliminary Statements. The definition of "Assumed Indebtedness" in the
         Preliminary Statements of the Contribution Agreement was inadvertently
         deleted in preparing the Contribution Agreement. The parties agree
         that, as used in the Contribution Agreement, the term "Assumed
         Indebtedness" shall mean (i) the $1.21 billion borrowed by TPI under
         the Term Loan Facilities and (ii) the $550 million promissory note
         issued by TPI to J.P. Morgan Securities, Inc. (the "Morgan Interim
         Note"), each of which will be assigned to and assumed by Newco at
         Closing.

                  2.  TPI, PCA, and Newco each hereby agree that the amount of
         the Term Loan Facilities and the Senior Subordinated Notes, and the
         terms of the Deferred-Pay Financing shall be on the terms set forth in
         the Offering Memorandum dated March 30, 1999,



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         notwithstanding that such amounts and terms differ from those set forth
         in the Contribution Agreement.

                  3.  TPI agrees that the Term Loan Facilities, pursuant to 
         which TPI will initially borrow $1.21 billion and which indebtedness
         will be assigned to and assumed by Newco as part of the Assumed
         Indebtedness, may, until the time of such assignment and assumption, be
         secured by certain depositary accounts and timberland assets of TPI, on
         terms satisfactory to TPI, notwithstanding that the Contribution
         Agreement provides for such loan to be unsecured as to TPI and that
         such security interest shall be released contemporaneously with the
         assignment and assumption by Newco.

                  4.  The parties agree that the Senior Subordinated Notes will
         not be issued by TPI, but that TPI will instead borrow $550 million
         pursuant to the Morgan Interim Note that will be assigned to and
         assumed by Newco as part of the Assumed Indebtedness, and which
         indebtedness will be repaid by Newco at the Closing.

                  5.  PCA hereby waives its right to elect, pursuant to Section
         5.16 of the Contribution Agreement, to instruct TPI to retain the
         Campbell Road Property, and agrees that the Campbell Road Property
         will, for purposes of the Contribution Agreement, constitute Owned Real
         Property and will be conveyed to Newco at Closing.

                  6.  Newco agrees that for a period of one year following the
         Closing Date TPI may (a) continue to use the PCA Marks on Corrugated
         Products purchased by TPI or its Affiliates from Newco pursuant to the
         Supply Agreements, until changes can be made to plates, molds, and
         similar items so as to allow Newco to produce such materials for TPI
         and such Affiliates without such PCA Marks, and (b) use the PCA marks
         on Corrugated Products that are in existence as of the Closing Date.
         Subject to the preceding sentence, TPI shall cease using the PCA Marks
         as soon as possible after Closing during such one year period and,
         following such one year period, TPI shall cease all use of any PCA
         Marks.

                  7.  PCA waives the condition to Closing set forth in Section
         6.2(g) of the Contribution Agreement, to the extent such closing
         condition would require PCA and Mr. Stecko to enter into any agreement
         beyond the letter agreement referred to therein, as such letter
         agreement may be modified.

                  8.  TPI hereby agrees and acknowledges that it has not 
         delivered a Dilution Notice pursuant to Paragraph E of the Preliminary
         Statements of the Contribution Agreement. PCA and TPI hereby agree
         that, notwithstanding anything in the Contribution Agreement to the
         contrary, upon issuance of Management Stock during the 120-day period
         following the Closing, Newco shall simultaneously redeem or purchase
         from PCA and TPI an aggregate number of Common Stock shares equal to
         the aggregate numbers of shares of Management Stock purchased during
         such 120-day period in a ratio of 55 shares from PCA to 45 shares from
         TPI at a price per share equal to the price per share paid by the
         Persons purchasing such Management Stock (provided such price per share
         is equal to the price per share paid for Common Stock purchased by PCA
         at Closing). 




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                  9.  The following changes are made to the definition of
         "Retained Liabilities": (A) paragraph (ix) is amended by adding the
         words "subject to paragraph (xiv) of this definition" after the word
         "Agreement" in clause (ii) thereof; and (B) a new paragraph (xiv) is
         added, a follows: "(xiv) all liability to make severance payments to
         seven named individuals who will be transferred to PCA and who have
         been identified to Newco and TPI in an aggregate amount of up to
         $385,000."

                  10. TPI has provided the Michigan Department of Natural
         Resources with a letter of credit in connection with certain operations
         at the Filer City Mill. TPI agrees to leave such letter of credit in
         place for 30 days after Closing or until Newco provides the Michigan
         Department of Natural Resources with a replacement letter of credit.
         Newco agrees to obtain and post such a replacement letter of credit
         within such 30-day period. Newco shall reimburse TPI for any draws made
         under TPI's letter of credit from and after Closing.

                  11. PCA and Newco hereby waive the closing condition set forth
         in Section 5.14(ii) of the Contribution Agreement, and TPI agrees at
         its sole expense to implement the steps set forth in Rick West's
         memorandum dated April 7, 1999, entitled "Form S-4 Exchange Option and
         Quarterly Filings," relating to the preparation of the quarterly
         financial statements referred to in Section 5.14(ii) of the
         Contribution Agreement provided that TPI hereby covenants it will
         deliver to PCA the financial statements referred to in Section 5.14(ii)
         of the Contribution Agreement (a) for the quarter ended March 31, 1998,
         no later than May 10, 1999 and (b) for each of the other quarters of
         1998, no later than May 31, 1999. Newco agrees that it will cause its
         appropriate financial officers and employees to provide reasonable
         assistance to TPI in its preparation of the financial statements
         referenced in this paragraph 11.

                  12. TPI hereby certifies that during the period from and
         including January 25, 1999 and the Closing, TPI has complied in all
         material respects with and not breached Section 5.2 of the Contribution
         Agreement.

                  13. TPI agrees to obtain, at its expense, for Newco commencing
         no later than the end of the term of the Technology, Financial and
         Administrative Transition Services Agreement (the "Transition
         Expiration Date"), licenses to use the following software, which
         licenses shall be substantially commensurate with the licenses to such
         software held by TPI or its Affiliates and used for the Containerboard
         Business prior to Closing (including, without limitation, as to scope
         and term as described in such existing licenses):

                  VENDOR                   NAME OF SOFTWARE
                  ------                   ----------------

                  Levi, Ray & Shoup        VPS and DRS

                  GEAC                     Financial Applications (GL, AR, AP, 
                                           FA)

                  Comshare                 System W




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                  Hyperion                 Hyperion (NT)

                  XRT                      Treasury Workstation (Netware)


                  14. TPI agrees to obtain, for Newco commencing no later than 
         the Transition Expiration Date, licenses to use the following software,
         which licenses shall be substantially commensurate with the licenses to
         such software held by TPI or its Affiliates and used for the
         Containerboard Business prior to Closing (including, without
         limitation, as to scope and term as described in such existing
         licenses):


                  VENDOR                   NAME OF SOFTWARE
                  ------                   ----------------

                  TSI                      Keymaster

                  Information Builders     Focus


         TPI shall pay 50% of the costs of obtaining such licenses and Newco
         shall pay 50% of the costs of obtaining such licenses.

                  15. TPI's sole obligation pursuant to paragraphs 13 and 14
         above shall be to purchase the licenses described in such paragraphs in
         the name of Newco, and shall not extend to any other fees, maintenance,
         costs, expenses or other payments required to be made pursuant to such
         licenses in respect of periods commencing after the Transition
         Expiration Date. The parties hereto hereby agree that neither TPI nor
         any of its Affiliates shall be required pursuant to the Contribution
         Agreement or any Ancillary Agreement to pay for any other license to
         use software that is not Related to the Containerboard Business but is
         used by TPI or its Affiliates to provide the services to Newco under
         the Transition Services Agreement, other than those licenses expressly
         described in paragraphs 13 or 14.

                  16. TPI , PCA and Newco hereby stipulate that the definition 
         of "Retained Liabilities" shall include all liabilities arising from,
         related to or incurred in connection with any state of facts or
         conditions or transactions (or series of facts, conditions or
         transactions) related, under or otherwise in connection with (i) IFC
         Credit Corporation v. Tenneco Packaging, Inc. filed in the Circuit
         Court of Cook County, Illinois 99CH4738 (the "Lawsuit") or (ii) the
         Master Lease Agreement between IFC Credit Corporation and TPI (f/k/a/
         Packaging Corporation of America) that is the subject of the Lawsuit,
         in each case other than liabilities to the extent arising from, related
         to or incurred in connection with any breach by Newco of its
         obligations under this paragraph 16. Newco agrees to cease using the
         equipment that is the subject of the Lawsuit (the "Equipment") and
         return the Equipment where directed by TPI as soon as reasonably
         practical, and in no event will Newco use the Equipment after (and it
         will return the Equipment by) June 30, 1999. Newco shall use its
         reasonable efforts consistent with TPI's past practice to maintain the
         Equipment in the 

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         operating condition and state of repair that it is in as of the date
         hereof, ordinary wear and tear excepted.

                  17. The parties hereby acknowledge that following the date of
         the Contribution Agreement and prior to the date hereof, approximately
         5,963 acres of timberland located in Hamilton, Dixie and Taylor
         Counties, Florida that were subject to the Existing Financing
         Arrangements have been sold (the "Florida Property Transfer"). The
         parties hereby agree that (i) no PCA Indemnified Party shall have,
         assert or be entitled to assert any claim (and each of PCA and Newco
         agrees that it shall not assert or permit to be asserted any claim)
         against TPI or any of its Subsidiaries or Affiliates arising out of, in
         connection with or related to the Florida Property Transfer, whether
         pursuant to the Contribution Agreement or otherwise and (ii) Newco
         assumes no liability with respect to the Florida Property Transfer.



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                  Please acknowledge your agreement to the foregoing by signing
below.

                                   Sincerely,

                                   TENNECO PACKAGING INC.


                                   By: /s/ James V. Faulkner, Jr.              
                                      ---------------------------------
                                      Its:  Vice President                    
                                           ---------------------------- 

Agreed to:

PCA HOLDINGS LLC



By: /s/ Samuel M. Mencoff     
   ------------------------  
       Samuel M. Mencoff
       Managing Director



PACKAGING CORPORATION OF AMERICA


By: /s/ Richard B. West         
   ------------------------     
   Its: Secretary                   
       --------------------          

Date:   April 12, 1999           
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