diff --git a/sexpjson.ml b/sexpjson.ml
new file mode 100644
index 0000000..5f1cf5a
--- /dev/null
+++ b/sexpjson.ml
@@ -0,0 +1,27 @@
+open Sexp
+
+let rec sexp_of_json j =
+  match j with
+  | Json.Num f -> Hint {hint = Str "num"; body = Str (Json.to_string j)}
+  | Json.Str s -> Str s
+  | Json.Arr js -> Arr (List.map sexp_of_json js)
+  | Json.Rec kvs -> Hint {hint = Str "obj";
+			  body = Arr (List.map (fun (k, v) -> Arr [Str k; sexp_of_json v]) kvs)}
+  | Json.Flg f -> Hint {hint = Str "bool"; body = Str (string_of_bool f)}
+  | Json.Nil -> Hint {hint = Str "null"; body = Arr []}
+
+let rec json_of_sexp x =
+  match x with
+  | Hint {hint = Str "num"; body = Str n} -> Json.Num (float_of_string n)
+  | Str s -> Json.Str s
+  | Arr xs -> Json.Arr (List.map json_of_sexp xs)
+  | Hint {hint = Str "obj"; body = Arr kvs} ->
+      Json.Rec (List.map
+		  (fun kv ->
+		    (match kv with
+		    | Arr [Str k; v] -> (k, json_of_sexp v)
+		    | _ -> syntax_error "Bad JSON-SEXP key-value"))
+		  kvs)
+  | Hint {hint = Str "bool"; body = Str bs} -> Json.Flg (bool_of_string bs)
+  | Hint {hint = Str "null"; body = Arr []} -> Json.Nil
+  | Hint {hint = h; body = b} -> Json.Rec ["_hint", json_of_sexp h; "_body", json_of_sexp b]