diff --git a/runtime/wasm/dune b/runtime/wasm/dune
index e9afd68d..4ec8e869 100644
--- a/runtime/wasm/dune
+++ b/runtime/wasm/dune
@@ -13,9 +13,19 @@
 
 (rule
  (target version-dependent.wat)
- (deps version-dependent/pre-5.2.wat)
+ (deps version-dependent/post-5.2.wat)
+ (enabled_if
+  (and
+   (>= %{ocaml_version} 5.1.0)
+   (< %{ocaml_version} 5.2.0)))
+ (action
+  (copy %{deps} %{target})))
+
+(rule
+ (target version-dependent.wat)
+ (deps version-dependent/pre-5.1.wat)
  (enabled_if
-  (< %{ocaml_version} 5.2.0))
+  (< %{ocaml_version} 5.1.0))
  (action
   (copy %{deps} %{target})))
 
diff --git a/runtime/wasm/marshal.wat b/runtime/wasm/marshal.wat
index 491d8438..46373b2b 100644
--- a/runtime/wasm/marshal.wat
+++ b/runtime/wasm/marshal.wat
@@ -48,8 +48,8 @@
       (func $caml_find_custom_operations
          (param (ref $string)) (result (ref null $custom_operations))))
 
-   (global $caml_marshal_header_size (export "caml_marshal_header_size")
-      (mut i32) (i32.const 20))
+   (import "version-dependent" "caml_marshal_header_size"
+      (global $caml_marshal_header_size i32))
 
    (global $input_val_from_string (ref $string)
       (array.new_fixed $string 21
diff --git a/runtime/wasm/version-dependent/pre-5.2.wat b/runtime/wasm/version-dependent/post-5.1.wat
similarity index 96%
rename from runtime/wasm/version-dependent/pre-5.2.wat
rename to runtime/wasm/version-dependent/post-5.1.wat
index 27c671a2..258505a5 100644
--- a/runtime/wasm/version-dependent/pre-5.2.wat
+++ b/runtime/wasm/version-dependent/post-5.1.wat
@@ -41,4 +41,6 @@
       (global.set $caml_domain_id (local.get $old))
       (drop (call $caml_ml_mutex_unlock (local.get $mutex)))
       (ref.i31 (local.get $id)))
+
+   (global (export "caml_marshal_header_size") i32 (i32.const 16))
 )
diff --git a/runtime/wasm/version-dependent/post-5.2.wat b/runtime/wasm/version-dependent/post-5.2.wat
index cd99832f..b4183d2d 100644
--- a/runtime/wasm/version-dependent/post-5.2.wat
+++ b/runtime/wasm/version-dependent/post-5.2.wat
@@ -54,4 +54,6 @@
             (ref.i31 (i32.const 0))
             (array.new_fixed $block 2 (ref.i31 (i32.const 0)) (local.get $res))))
       (ref.i31 (local.get $id)))
+
+   (global (export "caml_marshal_header_size") i32 (i32.const 16))
 )
diff --git a/runtime/wasm/version-dependent/pre-5.1.wat b/runtime/wasm/version-dependent/pre-5.1.wat
new file mode 100644
index 00000000..cc23b90a
--- /dev/null
+++ b/runtime/wasm/version-dependent/pre-5.1.wat
@@ -0,0 +1,46 @@
+;; Wasm_of_ocaml runtime support
+;; http://www.ocsigen.org/js_of_ocaml/
+;;
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU Lesser General Public License as published by
+;; the Free Software Foundation, with linking exception;
+;; either version 2.1 of the License, or (at your option) any later version.
+;;
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU Lesser General Public License for more details.
+;;
+;; You should have received a copy of the GNU Lesser General Public License
+;; along with this program; if not, write to the Free Software
+;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+
+(module
+   (import "obj" "caml_callback_1"
+      (func $caml_callback_1
+         (param (ref eq)) (param (ref eq)) (result (ref eq))))
+   (import "sync" "caml_ml_mutex_unlock"
+      (func $caml_ml_mutex_unlock (param (ref eq)) (result (ref eq))))
+   (import "domain" "caml_domain_latest_id"
+     (global $caml_domain_latest_id (mut i32)))
+   (import "domain" "caml_domain_id"
+     (global $caml_domain_id (mut i32)))
+
+   (func (export "caml_runtime_events_user_write")
+      (param (ref eq)) (param (ref eq)) (result (ref eq))
+      (ref.i31 (i32.const 0)))
+
+   (func (export "caml_domain_spawn")
+      (param $f (ref eq)) (param $mutex (ref eq)) (result (ref eq))
+      (local $id i32) (local $old i32)
+      (local.set $id (global.get $caml_domain_latest_id))
+      (global.set $caml_domain_latest_id
+         (i32.add (local.get $id) (i32.const 1)))
+      (local.set $old (global.get $caml_domain_id))
+      (drop (call $caml_callback_1 (local.get $f) (ref.i31 (i32.const 0))))
+      (global.set $caml_domain_id (local.get $old))
+      (drop (call $caml_ml_mutex_unlock (local.get $mutex)))
+      (ref.i31 (local.get $id)))
+
+   (global (export "caml_marshal_header_size") i32 (i32.const 20))
+)