<a href="{$OUTPUT_DIRECTORY}/{base_filename}.html">
    <xsl:value-of select="$OUTPUT_DIRECTORY" />
    <xsl:text>/</xsl:text>
    <xsl:value-of select="base_filename" />
    <xsl:text>.html</xsl:text>
  </a>

well this is a bit nearer what you want?

  <a href="{$OUTPUT_DIRECTORY}/{base_filename}.html">
    <xsl:value-of select=
         "concat($OUTPUT_DIRECTORY,'/',base_filename,'.html')" />
  </a>

David


 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list