diff --git a/gnu-system.am b/gnu-system.am
index 99e8e7dff4..1fdbd00731 100644
--- a/gnu-system.am
+++ b/gnu-system.am
@@ -696,6 +696,7 @@ dist_patch_DATA =						\
   gnu/packages/patches/synfig-build-fix.patch			\
   gnu/packages/patches/tar-d_ino_in_dirent-fix.patch		\
   gnu/packages/patches/tar-skip-unreliable-tests.patch		\
+  gnu/packages/patches/tcl-mkindex-deterministic.patch		\
   gnu/packages/patches/tcsh-fix-autotest.patch			\
   gnu/packages/patches/texi2html-document-encoding.patch	\
   gnu/packages/patches/texi2html-i18n.patch			\
diff --git a/gnu/packages/patches/tcl-mkindex-deterministic.patch b/gnu/packages/patches/tcl-mkindex-deterministic.patch
new file mode 100644
index 0000000000..2fcef6a4f7
--- /dev/null
+++ b/gnu/packages/patches/tcl-mkindex-deterministic.patch
@@ -0,0 +1,29 @@
+This patch ensures that the 'tclIndex' files generated by 'auto_mkindex'
+are sorted in a deterministic fashion.
+
+Fixes a non-determinism issue reported
+at <https://lists.gnu.org/archive/html/guix-devel/2015-10/msg00696.html>.
+
+--- tcl8.6.4/library/auto.tcl	2015-02-26 17:57:28.000000000 +0100
++++ tcl8.6.4/library/auto.tcl	2015-11-13 23:18:34.964831717 +0100
+@@ -207,6 +207,9 @@ proc auto_mkindex {dir args} {
+ 	set args *.tcl
+     }
+ 
++    # Keep file names sorted in a determistic order.
++    set args [lsort -ascii $args]
++
+     auto_mkindex_parser::init
+     foreach file [glob -- {*}$args] {
+ 	try {
+@@ -241,6 +244,10 @@ proc auto_mkindex_old {dir args} {
+     if {![llength $args]} {
+ 	set args *.tcl
+     }
++
++    # Keep file names sorted in a determistic order.
++    set args [lsort -ascii $args]
++
+     foreach file [glob -- {*}$args] {
+ 	set f ""
+ 	set error [catch {
diff --git a/gnu/packages/tcl.scm b/gnu/packages/tcl.scm
index c18b8b5112..6f6838c039 100644
--- a/gnu/packages/tcl.scm
+++ b/gnu/packages/tcl.scm
@@ -35,14 +35,14 @@
   (package
     (name "tcl")
     (version "8.6.4")
-    (source
-     (origin
-      (method url-fetch)
-      (uri (string-append "mirror://sourceforge/tcl/Tcl/"
-                          version "/tcl" version "-src.tar.gz"))
-      (sha256
-       (base32
-        "13cwa4bc85ylf5gfj9vk182lvgy60qni3f7gbxghq78wk16djvly"))))
+    (source (origin
+              (method url-fetch)
+              (uri (string-append "mirror://sourceforge/tcl/Tcl/"
+                                  version "/tcl" version "-src.tar.gz"))
+              (sha256
+               (base32
+                "13cwa4bc85ylf5gfj9vk182lvgy60qni3f7gbxghq78wk16djvly"))
+              (patches (list (search-patch "tcl-mkindex-deterministic.patch")))))
     (build-system gnu-build-system)
     (arguments
      '(#:phases (alist-cons-before