-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
Originator-Key-Asymmetric:
 MFgwCgYEVQgBAQICAf8DSgAwRwJAW2sNKK9AVtBzYZmr6aGjlWyK3XmZv3dTINen
 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 DhvVWx1kjZfJx4RsfjSAV8t8mY4aVMn8bHwzrKNNEsYW1zSIOsfGsmuTr33myfyp
 3MzksemU8dKh6TSmB11BnQ==

<SEC-DOCUMENT>0001331128-09-000003.txt : 20090313
<SEC-HEADER>0001331128-09-000003.hdr.sgml : 20090313
<ACCEPTANCE-DATETIME>20090313151847
ACCESSION NUMBER:		0001331128-09-000003
CONFORMED SUBMISSION TYPE:	D/A
PUBLIC DOCUMENT COUNT:		1
ITEM INFORMATION:		Rule 506
ITEM INFORMATION:		Investment Company Act Section 3(c)
ITEM INFORMATION:		Section 3(c)(7)
FILED AS OF DATE:		20090313
DATE AS OF CHANGE:		20090313
EFFECTIVENESS DATE:		20090313

FILER:

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			ACI Multi Strategy Market Neutral Master Fund LP
		CENTRAL INDEX KEY:			0001331128
		IRS NUMBER:				000000000
		STATE OF INCORPORATION:			E9
		FISCAL YEAR END:			1231

	FILING VALUES:
		FORM TYPE:		D/A
		SEC ACT:		1933 Act
		SEC FILE NUMBER:	021-78565
		FILM NUMBER:		09679738

	BUSINESS ADDRESS:	
		STREET 1:		C/O M&C CORP SVCES, LTD., P.O. BOX 309GT
		STREET 2:		UGLAND HOUSE, SOUTH CHURCH STREET
		CITY:			GRAND CAYMAN
		STATE:			E9
		ZIP:			0000
		BUSINESS PHONE:		1-345-949-8066

	MAIL ADDRESS:	
		STREET 1:		C/O M&C CORP SVCES, LTD., P.O. BOX 309GT
		STREET 2:		UGLAND HOUSE, SOUTH CHURCH STREET
		CITY:			GRAND CAYMAN
		STATE:			E9
		ZIP:			0000
</SEC-HEADER>
<DOCUMENT>
<TYPE>D/A
<SEQUENCE>1
<FILENAME>primary_doc.xml
<TEXT>
<XML>
<?xml version="1.0"?>
<edgarSubmission>

    <schemaVersion>X0201</schemaVersion>

    <submissionType>D/A</submissionType>

    <testOrLive>LIVE</testOrLive>

    <primaryIssuer>
        <cik>0001331128</cik>
        <entityName>ACI Multi Strategy Market Neutral Master Fund LP</entityName>
        <issuerAddress>
            <street1>C/O M&amp;C CORP SVCES, LTD., P.O. BOX 309GT</street1>
            <street2>UGLAND HOUSE, SOUTH CHURCH STREET</street2>
            <city>GRAND CAYMAN</city>
            <stateOrCountry>E9</stateOrCountry>
            <zipCode>0000</zipCode>
        </issuerAddress>
        <issuerPhoneNumber>1-345-949-8066</issuerPhoneNumber>
        <issuerPreviousNameList>
            <value>None</value>
        </issuerPreviousNameList>
        <jurisdictionOfInc>E9</jurisdictionOfInc>
        <entityType>Limited Partnership</entityType>
        <yearOfInc>
            <withinFiveYears>true</withinFiveYears>
            <value>2005</value>
        </yearOfInc>
    </primaryIssuer>

    <relatedPersonsList>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Peter</firstName>
                <middleName>Michael</middleName>
                <lastName>Algert</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>555 California Street</street1>
                <street2>Suite 3325</street2>
                <city>San Francisco</city>
                <stateOrCountry>CA</stateOrCountry>
                <zipCode>94104</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Kevin</firstName>
                <middleName>Karl</middleName>
                <lastName>Coldiron</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>555 California Street</street1>
                <street2>Suite 3325</street2>
                <city>San Francisco</city>
                <stateOrCountry>CA</stateOrCountry>
                <zipCode>94104</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Tommy</firstName>
                <middleName>Blaise</middleName>
                <lastName>Esperance Jr</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>555 California Street</street1>
                <street2>Suite 3325</street2>
                <city>San Francisco</city>
                <stateOrCountry>CA</stateOrCountry>
                <zipCode>94104</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
    </relatedPersonsList>

    <offeringData>
        <industryGroup>
            <industryGroupType>Pooled Investment Fund</industryGroupType>
            <investmentFundInfo>
                <investmentFundType>Hedge Fund</investmentFundType>
                <is40Act>true</is40Act>
            </investmentFundInfo>
        </industryGroup>
        <issuerSize>
            <aggregateNetAssetValueRange>Over $100,000,000</aggregateNetAssetValueRange>
        </issuerSize>
        <federalExemptionsExclusions>
            <item>06</item>
            <item>3C</item>
            <item>3C.7</item>
        </federalExemptionsExclusions>
        <typeOfFiling>
            <newOrAmendment>
                <isAmendment>true</isAmendment>
                <previousAccessionNumber>9999999997-05-029783</previousAccessionNumber>
            </newOrAmendment>
            <dateOfFirstSale>
                <value>2006-05-01</value>
            </dateOfFirstSale>
        </typeOfFiling>
        <durationOfOffering>
            <moreThanOneYear>true</moreThanOneYear>
        </durationOfOffering>
        <typesOfSecuritiesOffered>
            <isEquityType>true</isEquityType>
            <isPooledInvestmentFundType>true</isPooledInvestmentFundType>
        </typesOfSecuritiesOffered>
        <businessCombinationTransaction>
            <isBusinessCombinationTransaction>false</isBusinessCombinationTransaction>
            <clarificationOfResponse></clarificationOfResponse>
        </businessCombinationTransaction>
        <minimumInvestmentAccepted>1000000</minimumInvestmentAccepted>
        <salesCompensationList></salesCompensationList>
        <offeringSalesAmounts>
            <totalOfferingAmount>1000000000</totalOfferingAmount>
            <totalAmountSold>312620234</totalAmountSold>
            <totalRemaining>687379766</totalRemaining>
            <clarificationOfResponse></clarificationOfResponse>
        </offeringSalesAmounts>
        <investors>
            <hasNonAccreditedInvestors>false</hasNonAccreditedInvestors>
            <totalNumberAlreadyInvested>4</totalNumberAlreadyInvested>
        </investors>
        <salesCommissionsFindersFees>
            <salesCommissions>
                <dollarAmount>0</dollarAmount>
            </salesCommissions>
            <findersFees>
                <dollarAmount>0</dollarAmount>
            </findersFees>
            <clarificationOfResponse></clarificationOfResponse>
        </salesCommissionsFindersFees>
        <useOfProceeds>
            <grossProceedsUsed>
                <dollarAmount>0</dollarAmount>
            </grossProceedsUsed>
            <clarificationOfResponse></clarificationOfResponse>
        </useOfProceeds>
        <signatureBlock>
            <authorizedRepresentative>true</authorizedRepresentative>
            <signature>
                <issuerName>ACI Multi Strategy Market Neutral Master Fund LP</issuerName>
                <signatureName>Katy Royle Amara</signatureName>
                <nameOfSigner>Katy Royle Amara</nameOfSigner>
                <signatureTitle>Head of Operations</signatureTitle>
                <signatureDate>2009-03-13</signatureDate>
            </signature>
        </signatureBlock>
    </offeringData>
</edgarSubmission>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
-----END PRIVACY-ENHANCED MESSAGE-----