Exhibit 5

                 [WILLIAMS, MULLEN, CLARK & DOBBINS LETTERHEAD]



                                  July __, 1999


Board of Directors
Southern Financial Bancorp, Inc.
37 E. Main Street
Warrenton, Virginia  20186

Ladies and Gentlemen:

         This letter is in reference to the  Registration  Statement on Form S-4
dated July 1, 1999, filed by Southern  Financial  Bancorp,  Inc. (the "Company")
with the  Securities and Exchange  Commission  pursuant to the Securities Act of
1933, as amended (the  "Registration  Statement").  The  Registration  Statement
relates  to  1,045,734  shares of Common  Stock,  $0.01 par value per share (the
"Shares"),  which Shares are proposed to be offered to the  shareholders  of The
Horizon Bank of Virginia, a Virginia-chartered bank ("Horizon"),  pursuant to an
Agreement and Plan of  Reorganization,  dated as of May 3, 1999, as amended,  by
and between Horizon,  the Company,  and Southern  Financial Bank, a wholly owned
subsidiary  of the  Company,  and a related  Plan of Merger  (collectively,  the
"Agreement").

         We have examined such corporate  proceedings,  records and documents as
we considered  necessary  for the purposes of this opinion.  We have relied upon
certificates  of officers of the Company  where we have deemed it  necessary  in
connection with our opinion.

         Based upon such examination,  it is our opinion that the aforementioned
Shares, when issued against payment therefor pursuant to the Agreement,  will be
validly issued,  fully paid and nonassessable under the laws of the Commonwealth
of Virginia.

         We hereby  consent to the  filing of this  opinion as an exhibit to the
Registration Statement and to the reference to our firm under the caption "Legal
Opinion"  in the  Joint  Proxy  Statement  forming  a part  of the  Registration
Statement.

                                            Very truly yours,

                                            WILLIAMS, MULLEN, CLARK & DOBBINS
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