diff --git a/runtime/wasm/domain.wat b/runtime/wasm/domain.wat
index 2ba72bcf9..d07d70053 100644
--- a/runtime/wasm/domain.wat
+++ b/runtime/wasm/domain.wat
@@ -19,11 +19,6 @@
    (type $block (array (mut (ref eq))))
    (type $function_1 (func (param (ref eq) (ref eq)) (result (ref eq))))
    (type $closure (sub (struct (;(field i32);) (field (ref $function_1)))))
-   (import "sync" "caml_ml_mutex_unlock"
-      (func $caml_ml_mutex_unlock (param (ref eq)) (result (ref eq))))
-   (import "obj" "caml_callback_1"
-      (func $caml_callback_1
-         (param (ref eq)) (param (ref eq)) (result (ref eq))))
 
    (func (export "caml_atomic_cas")
       (param $ref (ref eq)) (param $o (ref eq)) (param $n (ref eq))
@@ -96,20 +91,9 @@
       (param (ref eq)) (result (ref eq))
       (ref.i31 (i32.const 1)))
 
-   (global $caml_domain_id (mut i32) (i32.const 0))
-   (global $caml_domain_latest_id (mut i32) (i32.const 1))
-
-   (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 $caml_domain_id (export "caml_domain_id") (mut i32) (i32.const 0))
+   (global $caml_domain_latest_id (export "caml_domain_latest_id") (mut i32)
+      (i32.const 1))
 
    (func (export "caml_ml_domain_id") (param (ref eq)) (result (ref eq))
       (ref.i31 (global.get $caml_domain_id)))
diff --git a/runtime/wasm/dune b/runtime/wasm/dune
index 2923b1ac9..4ec8e8693 100644
--- a/runtime/wasm/dune
+++ b/runtime/wasm/dune
@@ -3,6 +3,32 @@
  (package wasm_of_ocaml-compiler)
  (files runtime.wasm runtime.js))
 
+(rule
+ (target version-dependent.wat)
+ (deps version-dependent/post-5.2.wat)
+ (enabled_if
+  (>= %{ocaml_version} 5.2.0))
+ (action
+  (copy %{deps} %{target})))
+
+(rule
+ (target version-dependent.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} 5.1.0))
+ (action
+  (copy %{deps} %{target})))
+
 (rule
  (target runtime.wasm)
  (deps
diff --git a/runtime/wasm/marshal.wat b/runtime/wasm/marshal.wat
index 491d84385..46373b2bf 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/runtime_events.wat b/runtime/wasm/runtime_events.wat
index ff77e5570..8a50583cd 100644
--- a/runtime/wasm/runtime_events.wat
+++ b/runtime/wasm/runtime_events.wat
@@ -33,10 +33,6 @@
          (local.get $evtag)
          (local.get $evtype)))
 
-   (func (export "caml_runtime_events_user_write")
-      (param (ref eq)) (param (ref eq)) (param (ref eq)) (result (ref eq))
-      (ref.i31 (i32.const 0)))
-
    (func (export "caml_runtime_events_user_resolve")
       (param (ref eq)) (param (ref eq)) (param (ref eq)) (result (ref eq))
       (ref.i31 (i32.const 0)))
diff --git a/runtime/wasm/version-dependent/post-5.1.wat b/runtime/wasm/version-dependent/post-5.1.wat
new file mode 100644
index 000000000..258505a5e
--- /dev/null
+++ b/runtime/wasm/version-dependent/post-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 16))
+)
diff --git a/runtime/wasm/version-dependent/post-5.2.wat b/runtime/wasm/version-dependent/post-5.2.wat
new file mode 100644
index 000000000..b4183d2dc
--- /dev/null
+++ b/runtime/wasm/version-dependent/post-5.2.wat
@@ -0,0 +1,59 @@
+;; 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
+   (type $block (array (mut (ref eq))))
+   (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)) (param (ref eq)) (result (ref eq))
+      (ref.i31 (i32.const 0)))
+
+   (func (export "caml_domain_spawn")
+      (param $f (ref eq)) (param $term_sync_v (ref eq)) (result (ref eq))
+      (local $id i32) (local $old i32) (local $ts (ref $block)) (local $res (ref eq))
+      (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))
+      (local.set $res
+         (call $caml_callback_1 (local.get $f) (ref.i31 (i32.const 0))))
+      (global.set $caml_domain_id (local.get $old))
+      (local.set $ts (ref.cast (ref $block) (local.get $term_sync_v)))
+      (drop (call $caml_ml_mutex_unlock (array.get $block (local.get $ts) (i32.const 2))))
+      ;; TODO: fix exn case
+      (array.set
+         $block
+         (local.get $ts)
+         (i32.const 1)
+         (array.new_fixed
+            $block
+            2
+            (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 000000000..cc23b90ad
--- /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))
+)