diff --git a/CHANGES.md b/CHANGES.md
index be5de668c..68b4fa94d 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -2,7 +2,7 @@
 
 ## NEXT RELEASE
 
-- ...
+- #509: Change/Fix to use a symmetric barrier to synchronize domains
 
 ## 0.6
 
diff --git a/lib/STM_domain.ml b/lib/STM_domain.ml
index 18a883c2a..b00a36b51 100644
--- a/lib/STM_domain.ml
+++ b/lib/STM_domain.ml
@@ -25,9 +25,14 @@ module Make (Spec: Spec) = struct
   let run_par seq_pref cmds1 cmds2 =
     let sut = Spec.init_sut () in
     let pref_obs = interp_sut_res sut seq_pref in
-    let wait = Atomic.make true in
-    let dom1 = Domain.spawn (fun () -> while Atomic.get wait do Domain.cpu_relax() done; try Ok (interp_sut_res sut cmds1) with exn -> Error exn) in
-    let dom2 = Domain.spawn (fun () -> Atomic.set wait false; try Ok (interp_sut_res sut cmds2) with exn -> Error exn) in
+    let barrier = Atomic.make 2 in
+    let main cmds () =
+      Atomic.decr barrier;
+      while Atomic.get barrier <> 0 do Domain.cpu_relax() done;
+      try Ok (interp_sut_res sut cmds) with exn -> Error exn
+    in
+    let dom1 = Domain.spawn (main cmds1) in
+    let dom2 = Domain.spawn (main cmds2) in
     let obs1 = Domain.join dom1 in
     let obs2 = Domain.join dom2 in
     let ()   = Spec.cleanup sut in
diff --git a/lib/lin_domain.ml b/lib/lin_domain.ml
index 57eced2b6..1feafd82a 100644
--- a/lib/lin_domain.ml
+++ b/lib/lin_domain.ml
@@ -13,9 +13,14 @@ module Make_internal (Spec : Internal.CmdSpec [@alert "-internal"]) = struct
   let run_parallel (seq_pref,cmds1,cmds2) =
     let sut = Spec.init () in
     let pref_obs = interp sut seq_pref in
-    let wait = Atomic.make true in
-    let dom1 = Domain.spawn (fun () -> while Atomic.get wait do Domain.cpu_relax() done; try Ok (interp sut cmds1) with exn -> Error exn) in
-    let dom2 = Domain.spawn (fun () -> Atomic.set wait false; try Ok (interp sut cmds2) with exn -> Error exn) in
+    let barrier = Atomic.make 2 in
+    let main cmds () =
+      Atomic.decr barrier;
+      while Atomic.get barrier <> 0 do Domain.cpu_relax () done;
+      try Ok (interp sut cmds) with exn -> Error exn
+    in
+    let dom1 = Domain.spawn (main cmds1) in
+    let dom2 = Domain.spawn (main cmds2) in
     let obs1 = Domain.join dom1 in
     let obs2 = Domain.join dom2 in
     Spec.cleanup sut ;