diff --git a/src/Jitter2/World.Detect.cs b/src/Jitter2/World.Detect.cs
index d38c7a42..70b41b7d 100644
--- a/src/Jitter2/World.Detect.cs
+++ b/src/Jitter2/World.Detect.cs
@@ -24,7 +24,6 @@
 using System;
 using System.Diagnostics;
 using System.Runtime.CompilerServices;
-using System.Runtime.InteropServices;
 using Jitter2.Collision;
 using Jitter2.Collision.Shapes;
 using Jitter2.Dynamics;
@@ -417,30 +416,39 @@ private void Detect(IDynamicTreeProxy proxyA, IDynamicTreeProxy proxyB)
     public void GetArbiter(ulong id0, ulong id1, RigidBody b0, RigidBody b1, out Arbiter arbiter)
     {
         ArbiterKey arbiterKey = new(id0, id1);
+        bool exists = arbiters.TryGetValue(arbiterKey, out Arbiter? arb);
 
-        lock (arbiters)
+        if (!exists)
         {
-            ref Arbiter? arb = ref CollectionsMarshal.GetValueRefOrAddDefault(arbiters, arbiterKey, out bool exists);
-
-            if (!exists)
+            lock (memContacts)
             {
                 if (!Arbiter.Pool.TryPop(out arb))
                 {
                     arb = new Arbiter();
                 }
 
-                deferredArbiters.Add(arb);
+                bool success = arbiters.TryAdd(arbiterKey, arb);
 
-                var h = memContacts.Allocate(true);
-                arb.Handle = h;
-                h.Data.Init(b0, b1);
-                h.Data.Key = arbiterKey;
-                arb.Body1 = b0;
-                arb.Body2 = b1;
-            }
+                if (success)
+                {
+                    var h = memContacts.Allocate(true);
+                    arb.Handle = h;
+                    h.Data.Init(b0, b1);
+                    h.Data.Key = arbiterKey;
+                    arb.Body1 = b0;
+                    arb.Body2 = b1;
 
-            Debug.Assert(arb != null && memContacts.IsActive(arb.Handle));
-            arbiter = arb;
+                    Debug.Assert(arb != null && memContacts.IsActive(arb.Handle));
+
+                    deferredArbiters.Add(arb);
+                }
+                else
+                {
+                    Arbiter.Pool.Push(arb);
+                }
+            }
         }
+
+        arbiter = arb!;
     }
 }
\ No newline at end of file
diff --git a/src/Jitter2/World.Step.cs b/src/Jitter2/World.Step.cs
index aa47d481..1c0fee5d 100644
--- a/src/Jitter2/World.Step.cs
+++ b/src/Jitter2/World.Step.cs
@@ -466,7 +466,7 @@ private void RemoveBrokenArbiters()
 
                 memContacts.Free(handle);
                 IslandHelper.ArbiterRemoved(islands, arb);
-                arbiters.Remove(handle.Data.Key);
+                arbiters.TryRemove(handle.Data.Key, out _);
 
                 arb.Body1.RaiseEndCollide(arb);
                 arb.Body2.RaiseEndCollide(arb);
diff --git a/src/Jitter2/World.cs b/src/Jitter2/World.cs
index ecf68947..40472eac 100644
--- a/src/Jitter2/World.cs
+++ b/src/Jitter2/World.cs
@@ -24,6 +24,7 @@
 #pragma warning disable CS8618 // InitParallelCallbacks() - https://github.com/dotnet/roslyn/issues/32358
 
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Threading;
@@ -103,7 +104,7 @@ public SpanData(World world)
     /// </summary>
     public SpanData RawData => new(this);
 
-    private readonly Dictionary<ArbiterKey, Arbiter> arbiters = new();
+    private readonly ConcurrentDictionary<ArbiterKey, Arbiter> arbiters = new();
 
     private readonly ActiveList<Island> islands = new();
     private readonly ActiveList<RigidBody> bodies = new();
@@ -366,7 +367,7 @@ public void Remove(Arbiter arbiter)
         ActivateBodyNextStep(arbiter.Body2);
 
         IslandHelper.ArbiterRemoved(islands, arbiter);
-        arbiters.Remove(arbiter.Handle.Data.Key);
+        arbiters.TryRemove(arbiter.Handle.Data.Key, out _);
 
         brokenArbiters.Remove(arbiter.Handle);
         memContacts.Free(arbiter.Handle);