diff --git a/doc/sgml2html b/doc/sgml2html
index a5bbee9e..6072a7a3 100755
--- a/doc/sgml2html
+++ b/doc/sgml2html
@@ -34,7 +34,8 @@ $progs = {
   "GROFFMACRO" => "-ms",
   "AWK" => "/usr/share/linuxdoc-tools/awkwhich"
 };
-$ENV{"SGML_CATALOG_FILES"} = "$DataDir/dtd/catalog";
+$ENV{"SGML_CATALOG_FILES"} = "$DataDir/dtd/catalog" .
+  (defined $ENV{SGML_CATALOG_FILES} ? ":$ENV{SGML_CATALOG_FILES}" : "");
 
 require "$FindBin::Bin/LinuxDocTools.pm";
 &LinuxDocTools::init;
diff --git a/doc/sgml2latex b/doc/sgml2latex
index 02b60d94..c48f787d 100755
--- a/doc/sgml2latex
+++ b/doc/sgml2latex
@@ -34,7 +34,8 @@ $progs = {
   "GROFFMACRO" => "-ms",
   "AWK" => "/usr/share/linuxdoc-tools/awkwhich"
 };
-$ENV{"SGML_CATALOG_FILES"} = "$DataDir/dtd/catalog";
+$ENV{"SGML_CATALOG_FILES"} = "$DataDir/dtd/catalog" .
+  (defined $ENV{SGML_CATALOG_FILES} ? ":$ENV{SGML_CATALOG_FILES}" : "");
 
 require "$FindBin::Bin/LinuxDocTools.pm";
 &LinuxDocTools::init;
diff --git a/doc/sgml2txt b/doc/sgml2txt
index dfc017de..af0bcb49 100755
--- a/doc/sgml2txt
+++ b/doc/sgml2txt
@@ -34,7 +34,8 @@ $progs = {
   "GROFFMACRO" => "-ms",
   "AWK" => "/usr/share/linuxdoc-tools/awkwhich"
 };
-$ENV{"SGML_CATALOG_FILES"} = "$DataDir/dtd/catalog";
+$ENV{"SGML_CATALOG_FILES"} = "$DataDir/dtd/catalog" .
+  (defined $ENV{SGML_CATALOG_FILES} ? ":$ENV{SGML_CATALOG_FILES}" : "");
 
 require "$FindBin::Bin/LinuxDocTools.pm";
 &LinuxDocTools::init;