-----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,
 VIA0F45Len3lS1HR5dosTGUf8TusmdkXNyGgthxrfeYdlWSQB0FhDCZkCxYqYrq+
 eaG6LxXQUx50ZZ7kR8B9sA==

<SEC-DOCUMENT>0001140361-05-001379.txt : 20050215
<SEC-HEADER>0001140361-05-001379.hdr.sgml : 20050215
<ACCEPTANCE-DATETIME>20050215174303
ACCESSION NUMBER:		0001140361-05-001379
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20050211
FILED AS OF DATE:		20050215
DATE AS OF CHANGE:		20050215

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			SOUTHERN UNION CO
		CENTRAL INDEX KEY:			0000203248
		STANDARD INDUSTRIAL CLASSIFICATION:	NATURAL GAS DISTRIBUTION [4924]
		IRS NUMBER:				750571592
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		ONE PEI CENTER
		CITY:			WILKES-BARRE
		STATE:			PA
		ZIP:			18711
		BUSINESS PHONE:		(570) 820-2400

	MAIL ADDRESS:	
		STREET 1:		ONE PEI CENTER
		CITY:			WILKES-BARRE
		STATE:			PA
		ZIP:			18711

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			BRODSKY DAVID L
		CENTRAL INDEX KEY:			0001183999

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-06407
		FILM NUMBER:		05618457

	BUSINESS ADDRESS:	
		STREET 1:		259 CLARK AVE
		CITY:			PALM BEACH
		STATE:			FL
		ZIP:			33480
		BUSINESS PHONE:		5708202400

	MAIL ADDRESS:	
		STREET 1:		C/O SOUTHERN UNION CO
		STREET 2:		ONE PEI CENTER
		CITY:			WILKES BARRE
		STATE:			PA
		ZIP:			18711-0601
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>doc1.xml
<DESCRIPTION>FORM 4
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2005-02-11</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000203248</issuerCik>
        <issuerName>SOUTHERN UNION CO</issuerName>
        <issuerTradingSymbol>SUG</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001183999</rptOwnerCik>
            <rptOwnerName>BRODSKY DAVID L</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>259 CLARK AVE.</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>PALM BEACH</rptOwnerCity>
            <rptOwnerState>FL</rptOwnerState>
            <rptOwnerZipCode>33480</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Purchase Contract</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2005-02-11</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>4000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>50</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The purchase contract, together with a 5% undivided beneficial interest in $1,000 principal amount of a Southern Union senior note due February 16, 2008, comprise a Southern Union Corporate Unit.  The purchase contract obligates the Reporting Person to purchase from Southern Union, no later that February 16, 2008, for a price of $50 in cash, the following number of shares of Southern Union common stock, subject to anti-dilution adjustments:
	If the average closing price of Southern Union common stock over the 20-day period ending on the third trading day prior to February 16, 2008 equals or exceeds $30.7625, 1.6254 shares; if the average closing price of Southern Union common stock over the same period is less than $30.7625 but greater than $24.61, a number of shares having a value, based on the average closing price, equal to $50: and if the average closing price of Southern Union common stock over the same period is less than or equal to $24.61, 2.0317 shares.</footnote>
        <footnote id="F2">The Reporting Person also directly owns 36,750 shares of common stock and may be deemed to own 4,299 shares of common stock in the David L. Brodsky
Retirement Plan, by Van Liew Capital and Trust Company, as Trustee and 7,555 shares through the Issuer's Directors Deferred Compensation Plan. The Reporting Person disclaims beneficial ownership of these indirectly owned shares and this report  should not be deemed an admission that the Reporting Person is the beneficial owner of such shares for purposes of Section 16 or any other purpose.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Dennis K. Morgan for David L. Brodsky</signatureName>
        <signatureDate>2005-02-15</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
-----END PRIVACY-ENHANCED MESSAGE-----