EXHIBIT
      4





Conformed
        Copy










**ARCH
        CHEMICALS, INC.**
  **SECOND
        AMENDMENT TO NOTE PURCHASE AGREEMENT**


Dated
      as
      of May 12, 2006




Re:  Note
      Purchase Agreement dated as of March 20, 2002


and


$149,000,000
      Senior Notes, Series A, due March 20, 2007


$62,000,000
      Senior Notes, Series B, due March 20, 2009




To
      each
      of the Noteholders named


in
      Schedule I attached hereto




Ladies
      and Gentlemen:




Reference
      is made to the Note Purchase Agreement dated as of March 20, 2002 (said
      agreement as heretofore amended, the *“Note
      Purchase Agreement”* )
      among
      Arch Chemicals, Inc., a Virginia corporation (the *“Company”* ),
      and
      the institutional investors named in Schedule A attached thereto (the *“Noteholders”* ),
      under
      and pursuant to which $149,000,000 in aggregate principal amount of the Senior
      Notes, Series A, due March 20, 2007 (the *“Series
      A Notes”* )
      and
      $62,000,000 in aggregate principal amount of the Senior Notes, Series B, due
      March 20, 2009 (the *“Series
      B Notes,”* and
      together with the Series A Notes, the *“Notes”* )
      were
      originally issued and sold by the Company to the Noteholders. Terms used but
      not
      otherwise defined herein shall have the meanings set forth in the Note Purchase
      Agreement.




The
      Company hereby agrees with you in this Second Amendment to Note Purchase
      Agreement (the *“Second
      Amendment”* )
      as
      follows:


SECTION 1.
              AMENDMENTS
                  TO NOTE PURCHASE AGREEMENT.




*Section
      1.1.* *Amendment
      to Section 10.10 (Limitation on Securitization Transactions).* Section
      10.10 of the Note Purchase Agreement shall be and is hereby amended (i) by
      deleting the reference to “$80,000,000” therein and substituting “$100,000,000”
therefor, and (ii) by deleting the reference to “$145,000,000” therein and
      substituting “$180,000,000 therefor.




*Section
      1.2.* *Amendment
      to Schedule B (Definition of “Excess Securitization Assets”).* The
      definition of “Excess Securitization Assets” in Schedule B to the Note Purchase
      Agreement shall be and is hereby amended by deleting the reference to
“$100,000,000” therein and substituting “$135,000,000” therefor.




Arch Chemicals, Inc.
              
              Second
                  Amendment toNote
                  Purchase
                  Agreement


SECTION 2.
              REPRESENTATIONS
                  AND WARRANTIES




The
      Company represents and warrants that as of the date hereof:




*Section 2.1.* *Second
      Amendment is Legal and Authorized.* (a) The
      execution and delivery of the Second Amendment by the Company and compliance
      by
      the Company with all of the provisions of the Note Purchase Agreement, as
      amended by this Second Amendment —




(i) are
      within the corporate powers of the Company; and




(ii) will
      not
      violate any provisions of any law or any order of any court or governmental
      authority or agency and will not conflict with or result in any breach of any
      of
      the terms, conditions or provisions of, or constitute a default under the
      Articles of Incorporation or By-laws of the Company, or any indenture or other
      agreement or instrument to which the Company is a party or by which the Company
      may be bound or result in the imposition of any Liens or encumbrances on any
      property of the Company.




(b)  The
      execution and delivery of the Second Amendment has been duly authorized by
      all
      necessary corporate action on the part of the Company (no action by the
      stockholders of the Company being required by law, by the Articles of
      Incorporation or By-laws of the Company or otherwise, other than those actions
      which have been obtained or effected); and the Second Amendment has been duly
      executed and delivered by the Company, and the Note Purchase Agreement, as
      amended by the Second Amendment, constitutes the legal, valid and binding
      obligation, contract and agreement of the Company enforceable in accordance
      with
      its terms, subject to bankruptcy, insolvency, fraudulent conveyance and similar
      laws affecting creditors’ rights generally, and general principles of equity
      (regardless of whether the application of such principles is considered in
      a
      proceeding in equity or at law).




(c)  No
      consent, approval or authorization of, or registration, filing or declaration
      with, any Governmental Authority is required in connection with the execution,
      delivery or performance by the Company of this Second Amendment.




(d)  No
      Default or Event of Default exists under the Note Purchase
      Agreement.




SECTION
                  3.
              CONDITIONS
                  PRECEDENT TO EFFECTIVENESS OF THE SECOND AMENDMENT.




This
      Second Amendment shall become effective as of the date (the *“Effective
      Date”* )
      upon
      which the following conditions have been satisfied:




(a) The
      Required Holders shall have delivered an executed counterpart of this Second
      Amendment; and




(b) The
      Company shall have paid the fees, costs, expenses and disbursements of Chapman
      and Cutler LLP, special counsel to the Noteholders, incurred in connection
      with
      the consummation of the transactions contemplated by this Second
      Amendment.




-2-


Arch Chemicals, Inc.
            
            Second
                Amendment toNote
                Purchase
                Agreement




SECTION 4.
              MISCELLANEOUS




*Section 4.1.* *Ratification
      of Note Purchase Agreement.* Except
      as
      herein expressly amended, the Note Purchase Agreement is in all respects
      ratified and confirmed. If and to the extent that any of the terms or provisions
      of the Note Purchase Agreement is in conflict or inconsistent with any of the
      terms or provisions of this Second Amendment, this Second Amendment shall
      govern.




*Section 4.2.*  *No
      Legend Required.* References
      in the Note Purchase Agreement or in any Note, certificate, instrument or other
      document related to or delivered in connection with the transactions
      contemplated by the Note Purchase Agreement shall be deemed to be references
      to
      the Note Purchase Agreement as amended hereby and as further amended from time
      to time.




*Section 4.3.* *Successors
      and Assigns.* This
      Second Amendment shall be binding upon the Company and its respective successors
      and assigns and shall inure to the benefit of you and to the benefit of your
      successors and assigns, including each successive holder or holders of any
      Notes.




*Section 4.4.* *Counterparts.* This
      Second Amendment may be executed in any number of counterparts, each executed
      counterpart constituting an original but all together only one agreement.
      Signatures to this Second Amendment may be given by facsimile or other
      electronic transmission, and such signatures shall be effective as
      originals.




*Section
      4.5.* *Governing
      Law.* The
      Note
      Purchase Agreement as amended by this Second Amendment and the Notes shall
      be
      construed and enforced in accordance with, and the rights of the parties shall
      be governed by, the law of the State of New York excluding choice-of-law
      principles of the law of such State that would require the application of the
      laws of a jurisdiction other than such State.




*Signature
      Pages Follow*


-3-


Arch Chemicals, Inc.
            
            Second
                Amendment toNote
                Purchase
                Agreement




IN  WITNESS  WHEREOF, the
        Company has executed this Second Amendment to Note Purchase Agreement as
        of the
        day and year first above written.




ARCH
                CHEMICALS, INC.By:
            /s/ W.
              Paul
              BushName:
              W. Paul BushIts:
              Treasurer





Arch Chemicals, Inc.
            
            Second
                Amendment toNote
                Purchase
                Agreement




This
      Second Amendment to Note Purchase Agreement is accepted and agreed to as of
      the
      day and year first above written.





ING
                USA ANNUITY AND LIFE INSURANCECOMPANYRELIASTAR
                LIFE INSURANCE COMPANYSECURITY
                LIFE OF DENVER INSURANCECOMPANYBy:  ING Investment Management, LLC,
              asAgentBy:
                 /s/  Peter F. KomarekName:
                Peter F. KomarekTitle:
                Vice President





Arch Chemicals, Inc.
            
            Second
                Amendment toNote
                Purchase
                Agreement




This
      Second Amendment to Note Purchase Agreement is accepted and agreed to as of
      the
      day and year first above written.





ALLSTATE
                LIFE INSURANCE COMPANYBy:
            /s/ Robert
              B.
              BodettName: Robert B.
              Bodett


By:
            /s/ Jerry
              D.
              ZinkulaName: Jerry D. ZinkulaAuthorized
              Signatories


ALLSTATE
                LIFE INSURANCE COMPANY OFNEW
                YORKBy:
            /s/ Robert
              B.
              BodettName: Robert B.
              Bodett


By:
            /s/ Jerry
              D.
              ZinkulaName: Jerry D. ZinkulaAuthorized
              Signatories







Arch Chemicals, Inc.
            
            Second
                Amendment toNote
                Purchase
                Agreement




This
      Second Amendment to Note Purchase Agreement is accepted and agreed to as of
      the
      day and year first above written.





METROPOLITAN
                LIFE INSURANCE COMPANYBy:
            /s/ Frank
              O.
              MonfalconeName: Frank O. MonfalconeTitle:
              Director





Arch Chemicals, Inc.
            
            Second
                Amendment toNote
                Purchase
                Agreement




This
      Second Amendment to Note Purchase Agreement is accepted and agreed to as of
      the
      day and year first above written.





NEW
                YORK LIFE INSURANCE COMPANYBy:
            /s/ Kathleen
              A. HaberkernName: Kathleen A. HaberkernTitle:
              Investment Vice President NEW
                YORK LIFE INSURANCE AND ANNUITYCORPORATIONBy   New
              York Life Investment ManagementLLC,
                Its Investment ManagerBy: /s/ Kathleen
                A.
                HaberkernName:
                Kathleen A. HaberkernTitle:
                DirectorNEW
                YORK LIFE INSURANCE AND ANNUITYCORPORATIONINSTITUTIONALLY
              OWNED LIFE INSURANCESEPARATE
                ACCOUNTByNew
              York Life Investment ManagementLLC,
                its Investment ManagerBy:
 /s/
                Kathleen A. HaberkernName:
                Kathleen A. HaberkernTitle:
                Director







Arch Chemicals, Inc.
            
            Second
                Amendment toNote
                Purchase
                Agreement


This
      Second Amendment to Note Purchase Agreement is accepted and agreed to as of
      the
      day and year first above written.





THE
                OHIO NATIONAL LIFE INSURANCE COMPANYBy:
            /s/ Jed
              R.
              MartinName: Jed R. MartinTitle:
              Vice President, Private Placements





SCHEDULE
        I


ING
      USA
      Annuity and Life Insurance Company


c/o
      ING
      Investment Management LLC


Minneapolis,
      Minnesota 55401-2121





Security
      Life of Denver Insurance Company


c/o
      ING
      Investment Management LLC


Minneapolis,
      Minnesota 55401-2121





ReliaStar
      Life Insurance Company


c/o
      ING
      Investment Management LLC


Minneapolis,
      Minnesota 55401-2121





Metropolitan
      Life Insurance Company


New
      York,
      New York 10010-3690





New
      York
      Life Insurance Company


c/o
      New
      York Life Investment Management LLC


New
      York,
      New York 10010





New
      York
      Life Insurance and Annuity Corporation


c/o
      New
      York Life Investment Management LLC


New
      York,
      New York 10010





New
      York
      Life Insurance and Annuity Corporation


Institutionally
      Owned Life Insurance Separate Account


c/o
      New
      York Life Investment Management LLC


New
      York,
      New York 10010





Nationwide
      Life Insurance Company


Columbus,
      Ohio 43215-2220





Nationwide
      Life and Annuity Insurance Company


Columbus,
      Ohio 43215-2220





The
      Canada Life Assurance Company


Winnipeg,
      Manitoba


Canada
      R3C 3A5





Canada
      Life Insurance Company of America


c/o
      Great-West Life & Annuity Insurance Company


Greenwood,
      Colorado 80111




SCHEDULE
      I  (to
        Second Amendment to Note Purchase Agreement)





Erie
      Indemnity Company


Erie,
      Pennsylvania 16530





Erie
      Family Life Insurance Company


Erie,
      Pennsylvania 16530





Erie
      Indemnity Company, as attorney in fact,


for
      Erie
      Insurance Exchange


Erie,
      Pennsylvania 16530





Erie
      Insurance Group Retirement Trust Company


Erie,
      Pennsylvania 16530





The
      Ohio
      National Life Insurance Company


Cincinnati,
      Ohio 45242





Allstate
      Life Insurance Company


Northbrook,
      Illinois 60062-7127





Allstate
      Life Insurance Company of New York


Northbrook,
      Illinois 60062-7127





Beneficial
      Life Insurance Company


Salt
      Lake
      City, Utah 84136




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