diff --git a/src/client.ml b/src/client.ml
index b9a99d86086..e804a2c33bf 100644
--- a/src/client.ml
+++ b/src/client.ml
@@ -337,6 +337,28 @@ let get_archive t nv =
   Filename.link src dst;
   dst
 
+(* Return the contents of a fully qualified variable *)
+let contents_of_variable t v =
+  let name = Full_variable.package v in
+  let var = Full_variable.variable v in
+  let _nv =
+    try find_package_from_name name t.installed
+    with Not_found ->
+      Globals.error_and_exit "Package %s is not installed" (N.to_string name) in
+  let c = File.Dot_config.safe_read (Path.C.config t.compiler name) in
+  try match Full_variable.section v with
+  | None   -> File.Dot_config.variable c var
+  | Some s -> File.Dot_config.Sections.variable c s var
+  with Not_found ->
+    Globals.error_and_exit "%s is not defined" (Full_variable.to_string v)
+
+(* Substitute the file contents *)
+let substitute_file t f =
+  let src = Filename.add_extension f "in" in
+  let contents = File.Subst.read src in
+  let newcontents = File.Subst.replace contents (contents_of_variable t) in
+  File.Subst.write f newcontents
+
 let proceed_tochange t nv_old nv =
   (* First, uninstall any previous version *)
   (match nv_old with 
@@ -348,8 +370,15 @@ let proceed_tochange t nv_old nv =
   Dirname.rmdir p_build;
   Filename.extract (get_archive t nv) p_build;
 
-  (* Call the build script and copy the output files *)
+  (* OPAM files should be read in the right directory to get the
+     correct absolute path for the substitution files *)
+  Dirname.chdir (Path.C.build t.compiler nv);
   let opam = File.OPAM.read (Path.G.opam t.global nv) in
+
+  (* Substitute the configuration files *)
+  List.iter (substitute_file t) (File.OPAM.substs opam);
+
+  (* Call the build script and copy the output files *)
   let commands =
     List.map
       (fun cmd -> String.concat " " (List.map (Printf.sprintf "'%s'") cmd))
@@ -574,10 +603,12 @@ let upload upload repo =
   Filename.remove upload_descr;
   Filename.remove upload_archives
 
+module FC = File.Dot_config
+
 let config request =
   log "config %s" (string_of_config request);
   let t = load_state () in
-  let module FC = File.Dot_config in
+
   match request with
   (* List all the available variables *)
   | List_vars ->
@@ -608,21 +639,13 @@ let config request =
           (Full_variable.to_string fv)
           (string_of_variable_contents contents)
       ) (List.rev variables)
+
   | Variable v ->
-      let name = Full_variable.package v in
-      let var = Full_variable.variable v in
-      let _nv =
-        try find_package_from_name name t.installed
-        with Not_found ->
-          Globals.error_and_exit "Package %s is not installed" (N.to_string name) in
-      let c = FC.safe_read (Path.C.config t.compiler name) in
-      let contents =
-        try match Full_variable.section v with
-          | None   -> FC.variable c var
-          | Some s -> FC.Sections.variable c s var
-        with Not_found ->
-          Globals.error_and_exit "%s is not defined" (Full_variable.to_string v) in
+      let contents = contents_of_variable t v in
       Globals.msg "%s\n" (string_of_variable_contents contents)
+
+  | Subst fs -> List.iter (substitute_file t) fs
+          
   | _ -> failwith "TODO"
             
 (*
diff --git a/src/file.ml b/src/file.ml
index a8f8f3ba244..675068e829e 100644
--- a/src/file.ml
+++ b/src/file.ml
@@ -594,6 +594,48 @@ module Dot_config = struct
   module Sections = MK (struct let get t = t.sections end)
 end
 
+module Subst = struct
+
+  let internal = "subst"
+
+  type t = Raw.t
+
+  let empty = Raw.of_string ""
+
+  let of_string filename str = str
+
+  let to_string filename t = t
+
+  let pcre_replace ~pat ~templ str =
+    let r = Re_perl.compile_pat pat in
+    let b = Buffer.create 1024 in
+    let rec loop pos =
+      if pos >= String.length str then
+        Buffer.contents b
+      else if Re.execp ~pos r str then (
+        let ss = Re.exec ~pos r str in
+        let start, fin = Re.get_ofs ss 0 in
+        let pat = Re.get ss 0 in
+        Buffer.add_substring b str pos (start - pos);
+        Buffer.add_string b (templ pat);
+        loop fin
+      ) else (
+        Buffer.add_substring b str pos (String.length str - pos);
+        loop (String.length str)
+      )
+    in
+    loop 0
+
+  let replace t f =
+    let templ str =
+      let str = String.sub str 2 (String.length str - 4) in
+      let v = Full_variable.of_string str in
+      string_of_variable_contents (f v) in
+    let str = pcre_replace ~pat:"%\\{[^%]+\\}%" ~templ (Raw.to_string t) in
+    Raw.of_string str
+
+end
+
 end
 
 module type F = sig
@@ -690,3 +732,8 @@ module Updated = struct
   include Updated
   include Make (Updated)
 end
+
+module Subst = struct
+  include Subst
+  include Make (Subst)
+end
diff --git a/src/file.mli b/src/file.mli
index aa4a434f5fe..39b238eb5be 100644
--- a/src/file.mli
+++ b/src/file.mli
@@ -176,3 +176,15 @@ module Repo_index: IO_FILE with type t = string N.Map.t
 
 (** Repository config: [$opam/repo/$repo/config] *)
 module Repo_config: IO_FILE with type t = repository
+
+(** {2 Substitution files} *)
+
+(** Substitution files *)
+module Subst: sig
+
+  include IO_FILE
+
+  (** Substitute the variable appearing in a file *)
+  val replace: t -> (full_variable -> variable_contents) -> t
+
+end
diff --git a/src/opam.ml b/src/opam.ml
index 03560db8ecd..43a827ac2df 100644
--- a/src/opam.ml
+++ b/src/opam.ml
@@ -112,6 +112,7 @@ let specs = [
   ("-asmlink"  , Arg.Unit (set `Asmlink) , " Display native link options");
   ("-list-vars", Arg.Unit (set `ListVars), " Display the contents of all available variables");
   ("-var"      , Arg.Unit (set `Var)     , " Display the content of a variable");
+  ("-subst"    , Arg.Unit (set `Subst)   , " Substitute variables in files");
 ]
 let args n =
   (* XXX: big hack *)
@@ -142,7 +143,8 @@ let config = {
       | Some `Var when List.length names = 1
                        -> Variable (Full_variable.of_string (List.hd names))
       | Some `Var      ->
-          bad_argument "config" "-var takes exactly one parameter" in
+          bad_argument "config" "-var takes exactly one parameter"
+      | Some `Subst    -> Subst (List.map Filename.of_string names) in
     Client.config config
 }
 
diff --git a/src/types.ml b/src/types.ml
index 885a3ef199c..eaa6542d76b 100644
--- a/src/types.ml
+++ b/src/types.ml
@@ -91,6 +91,7 @@ module Filename: sig
   val write: t -> Raw.t -> unit
   val exists: t -> bool
   val check_suffix: t -> string -> bool
+  val add_extension: t -> string -> t
   val list: dirname -> t list
   val with_raw: (Raw.t -> 'a) -> t -> 'a
   val copy_in: t -> dirname -> unit
@@ -140,6 +141,9 @@ end = struct
   let check_suffix filename s =
     F.check_suffix (to_string filename) s
 
+  let add_extension filename suffix =
+    of_string ((to_string filename) ^ "." ^ suffix)
+
   let list d =
     let fs = Run.files (Dirname.to_string d) in
     List.map of_string fs
@@ -472,7 +476,7 @@ type config =
   | List_vars
   | Variable of full_variable
   | Compil   of rec_config_option
-  | Subst    of Filename.t list
+  | Subst    of filename list
 
 let p msg l =
   Printf.sprintf "%s %s"
diff --git a/src/types.mli b/src/types.mli
index 29b76924ab4..ca9350a9c21 100644
--- a/src/types.mli
+++ b/src/types.mli
@@ -105,6 +105,9 @@ module Filename: sig
   (** Check whether a file has a given suffix *)
   val check_suffix: t -> string -> bool
 
+  (** Remove the file suffix *)
+  val add_extension: t -> string -> t
+
   (** List all the filenames (ie. which are not directories) in a directory *)
   val list: dirname -> t list