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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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<SEC-DOCUMENT>0000859911-05-000015.txt : 20051229
<SEC-HEADER>0000859911-05-000015.hdr.sgml : 20051229
<ACCEPTANCE-DATETIME>20051229165826
ACCESSION NUMBER:		0000859911-05-000015
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20051221
FILED AS OF DATE:		20051229
DATE AS OF CHANGE:		20051229

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			SOUTHWEST ROYALTIES INSTITUTIONAL INCOME FUND X-B LP
		CENTRAL INDEX KEY:			0000859911
		STANDARD INDUSTRIAL CLASSIFICATION:	CRUDE PETROLEUM & NATURAL GAS [1311]
		IRS NUMBER:				752332174
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		6 DESTA DRIVE, SUITE 1100
		CITY:			MIDLAND
		STATE:			TX
		ZIP:			79705
		BUSINESS PHONE:		4326826324

	MAIL ADDRESS:	
		STREET 1:		6 DESTA DRIVE, SUITE 1100
		CITY:			MIDLAND
		STATE:			TX
		ZIP:			79705

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			CLAYTON WILLIAMS ENERGY INC /DE
		CENTRAL INDEX KEY:			0000880115
		STANDARD INDUSTRIAL CLASSIFICATION:	CRUDE PETROLEUM & NATURAL GAS [1311]
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-19601
		FILM NUMBER:		051292002

	BUSINESS ADDRESS:	
		STREET 1:		SIX DESTA DR
		STREET 2:		STE 6500
		CITY:			MIDLAND
		STATE:			TX
		ZIP:			79705
		BUSINESS PHONE:		9156826324

	MAIL ADDRESS:	
		STREET 1:		SIX DESTA DRIVE
		STREET 2:		STE 6500
		CITY:			MIDLAND
		STATE:			TX
		ZIP:			79705
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>primary_doc.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2005-12-21</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000859911</issuerCik>
        <issuerName>SOUTHWEST ROYALTIES INSTITUTIONAL INCOME FUND X-B LP</issuerName>
        <issuerTradingSymbol>N/A</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0000880115</rptOwnerCik>
            <rptOwnerName>CLAYTON WILLIAMS ENERGY INC /DE</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>SIX DESTA DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2>STE 6500</rptOwnerStreet2>
            <rptOwnerCity>MIDLAND</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>79705</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Limited Partnership Interest</value>
            </securityTitle>
            <transactionDate>
                <value>2005-12-21</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>6</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>150.59</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1367</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F1"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>*</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Southwest Royalties, Inc. the Managing General Partner of the Issuer is a wholly owned subsidiary of Clayton Williams Energy, Inc.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ L. Paul Latham, signed on behalf of Clayton Williams Energy, Inc. by L. Paul Latham, Executive Vice President and Chief Operating Officer</signatureName>
        <signatureDate>2005-12-29</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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