-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
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 A/JwbQb1iZdwaU9pkLOzVA==

<SEC-DOCUMENT>0000859911-05-000003.txt : 20050316
<SEC-HEADER>0000859911-05-000003.hdr.sgml : 20050316
<ACCEPTANCE-DATETIME>20050316143113
ACCESSION NUMBER:		0000859911-05-000003
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20050315
FILED AS OF DATE:		20050316
DATE AS OF CHANGE:		20050316

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:		407 N BIG SPRING STE 300
		CITY:			MIDLAND
		STATE:			TX
		ZIP:			79701
		BUSINESS PHONE:		9156869927

	MAIL ADDRESS:	
		STREET 1:		407 N BIG SPRING ST SUITE 300
		CITY:			MIDLAND
		STATE:			TX
		ZIP:			79701

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			SOUTHWEST ROYALTIES INC
		CENTRAL INDEX KEY:			0000821699
		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:		05684768

	BUSINESS ADDRESS:	
		STREET 1:		407 N BIG SPRING STREET SUITE 300
		CITY:			MIDLAND
		STATE:			TX
		ZIP:			79701
		BUSINESS PHONE:		9156869927

	MAIL ADDRESS:	
		STREET 1:		407 N BIG SPRING STREET SUITE 300
		CITY:			MIDLAND
		STATE:			TX
		ZIP:			79701
</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>2005-03-15</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>0000821699</rptOwnerCik>
            <rptOwnerName>SOUTHWEST ROYALTIES INC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>6 DESTA DRIVE, SUITE 6500</rptOwnerStreet1>
            <rptOwnerStreet2></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>2005-03-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>62</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>74.31</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1244</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <ownerSignature>
        <signatureName>/s/ L. Paul Latham, signed on behalf of Southwest Royalties, Inc. by L. Paul Latham, President and Chief Executive Officer</signatureName>
        <signatureDate>2005-03-16</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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