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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
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<SEC-DOCUMENT>0001488625-10-000002.txt : 20100901
<SEC-HEADER>0001488625-10-000002.hdr.sgml : 20100901
<ACCEPTANCE-DATETIME>20100901144242
ACCESSION NUMBER:		0001488625-10-000002
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:		20100901
DATE AS OF CHANGE:		20100901
EFFECTIVENESS DATE:		20100901

FILER:

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			EnerVest Energy Institutional Fund XII-A, L.P.
		CENTRAL INDEX KEY:			0001488625
		IRS NUMBER:				010938041
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	FILING VALUES:
		FORM TYPE:		D/A
		SEC ACT:		1933 Act
		SEC FILE NUMBER:	021-142196
		FILM NUMBER:		101052294

	BUSINESS ADDRESS:	
		STREET 1:		1001 FANNIN
		STREET 2:		SUITE 800
		CITY:			HOUSTON
		STATE:			TX
		ZIP:			77002
		BUSINESS PHONE:		713-659-3500

	MAIL ADDRESS:	
		STREET 1:		1001 FANNIN
		STREET 2:		SUITE 800
		CITY:			HOUSTON
		STATE:			TX
		ZIP:			77002
</SEC-HEADER>
<DOCUMENT>
<TYPE>D/A
<SEQUENCE>1
<FILENAME>primary_doc.xml
<TEXT>
<XML>
<?xml version="1.0"?>
<edgarSubmission>

    <schemaVersion>X0704</schemaVersion>

    <submissionType>D/A</submissionType>

    <testOrLive>LIVE</testOrLive>

    <primaryIssuer>
        <cik>0001488625</cik>
        <entityName>EnerVest Energy Institutional Fund XII-A, L.P.</entityName>
        <issuerAddress>
            <street1>1001 FANNIN</street1>
            <street2>SUITE 800</street2>
            <city>HOUSTON</city>
            <stateOrCountry>TX</stateOrCountry>
            <stateOrCountryDescription>TEXAS</stateOrCountryDescription>
            <zipCode>77002</zipCode>
        </issuerAddress>
        <issuerPhoneNumber>(713) 659-3500</issuerPhoneNumber>
        <jurisdictionOfInc>DELAWARE</jurisdictionOfInc>
        <issuerPreviousNameList>
            <value>None</value>
        </issuerPreviousNameList>
        <edgarPreviousNameList>
            <value>None</value>
        </edgarPreviousNameList>
        <entityType>Limited Partnership</entityType>
        <yearOfInc>
            <withinFiveYears>true</withinFiveYears>
            <value>2009</value>
        </yearOfInc>
    </primaryIssuer>

    <relatedPersonsList>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>N/A</firstName>
                <lastName>EnerVest, Ltd.</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>1001 Fannin</street1>
                <street2>Suite 800</street2>
                <city>Houston</city>
                <stateOrCountry>TX</stateOrCountry>
                <stateOrCountryDescription>TEXAS</stateOrCountryDescription>
                <zipCode>77002</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification>General Partner of the Issuer</relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>N/A</firstName>
                <lastName>EnerVest Management GP, L.C.</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>1001 Fannin</street1>
                <street2>Suite 800</street2>
                <city>Houston</city>
                <stateOrCountry>TX</stateOrCountry>
                <stateOrCountryDescription>TEXAS</stateOrCountryDescription>
                <zipCode>77002</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification>General Partner of the General Partner of the Issuer</relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>James</firstName>
                <lastName>Vanderhider</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>1001 Fannin</street1>
                <street2>Suite 800</street2>
                <city>Houston</city>
                <stateOrCountry>TX</stateOrCountry>
                <stateOrCountryDescription>TEXAS</stateOrCountryDescription>
                <zipCode>77002</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification>Principal of the General Partner of the General Partner of the Issuer</relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Mark</firstName>
                <lastName>Houser</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>1001 Fannin</street1>
                <street2>Suite 800</street2>
                <city>Houston</city>
                <stateOrCountry>TX</stateOrCountry>
                <stateOrCountryDescription>TEXAS</stateOrCountryDescription>
                <zipCode>77002</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification>Principal of the General Partner of the General Partner of the Issuer</relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>John</firstName>
                <lastName>Walker</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>1001 Fannin</street1>
                <street2>Suite 800</street2>
                <city>Houston</city>
                <stateOrCountry>TX</stateOrCountry>
                <stateOrCountryDescription>TEXAS</stateOrCountryDescription>
                <zipCode>77002</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification>Principal of the General Partner of the General Partner of the Issuer</relationshipClarification>
        </relatedPersonInfo>
    </relatedPersonsList>

    <offeringData>
        <industryGroup>
            <industryGroupType>Pooled Investment Fund</industryGroupType>
            <investmentFundInfo>
                <investmentFundType>Other Investment Fund</investmentFundType>
                <is40Act>false</is40Act>
            </investmentFundInfo>
        </industryGroup>
        <issuerSize>
            <aggregateNetAssetValueRange>Decline to Disclose</aggregateNetAssetValueRange>
        </issuerSize>
        <federalExemptionsExclusions>
            <item>06</item>
            <item>3C</item>
            <item>3C.7</item>
        </federalExemptionsExclusions>
        <typeOfFiling>
            <newOrAmendment>
                <isAmendment>true</isAmendment>
                <previousAccessionNumber>0001488625-10-000001</previousAccessionNumber>
            </newOrAmendment>
            <dateOfFirstSale>
                <value>2010-05-07</value>
            </dateOfFirstSale>
        </typeOfFiling>
        <durationOfOffering>
            <moreThanOneYear>true</moreThanOneYear>
        </durationOfOffering>
        <typesOfSecuritiesOffered>
            <isEquityType>true</isEquityType>
            <isPooledInvestmentFundType>true</isPooledInvestmentFundType>
        </typesOfSecuritiesOffered>
        <businessCombinationTransaction>
            <isBusinessCombinationTransaction>false</isBusinessCombinationTransaction>
            <clarificationOfResponse></clarificationOfResponse>
        </businessCombinationTransaction>
        <minimumInvestmentAccepted>0</minimumInvestmentAccepted>
        <salesCompensationList></salesCompensationList>
        <offeringSalesAmounts>
            <totalOfferingAmount>Indefinite</totalOfferingAmount>
            <totalAmountSold>289500000</totalAmountSold>
            <totalRemaining>Indefinite</totalRemaining>
            <clarificationOfResponse></clarificationOfResponse>
        </offeringSalesAmounts>
        <investors>
            <hasNonAccreditedInvestors>false</hasNonAccreditedInvestors>
            <totalNumberAlreadyInvested>22</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>false</authorizedRepresentative>
            <signature>
                <issuerName>EnerVest Energy Institutional Fund XII-A, L.P.</issuerName>
                <signatureName>James Vanderhider</signatureName>
                <nameOfSigner>James Vanderhider</nameOfSigner>
                <signatureTitle>Principal of General Partner of General Partner of Issuer</signatureTitle>
                <signatureDate>2010-08-31</signatureDate>
            </signature>
        </signatureBlock>
    </offeringData>
</edgarSubmission>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
-----END PRIVACY-ENHANCED MESSAGE-----