diff --git a/src/benchmark/Akka.Benchmarks/Cluster/ReachabilityBenchmarks.cs b/src/benchmark/Akka.Benchmarks/Cluster/ReachabilityBenchmarks.cs
index 32869c18869..fc8362a2bd8 100644
--- a/src/benchmark/Akka.Benchmarks/Cluster/ReachabilityBenchmarks.cs
+++ b/src/benchmark/Akka.Benchmarks/Cluster/ReachabilityBenchmarks.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Collections.Immutable;
 using System.Linq;
 using Akka.Util;
 using Akka.Actor;
@@ -13,7 +14,7 @@ namespace Akka.Benchmarks.Cluster
     [Config(typeof(MicroBenchmarkConfig))]
     public class ReachabilityBenchmarks
     {
-        [Params(10, 100, 250)]
+        [Params(100)]
         public int NodesSize;
 
         [Params(100)]
@@ -51,7 +52,7 @@ private Reachability AddUnreachable(Reachability baseReachability, int count)
         internal Reachability Reachability1;
         internal Reachability Reachability2;
         internal Reachability Reachability3;
-        internal HashSet<UniqueAddress> Allowed;
+        internal ImmutableHashSet<UniqueAddress> Allowed;
 
         [GlobalSetup]
         public void Setup()
@@ -59,7 +60,7 @@ public void Setup()
             Reachability1 = CreateReachabilityOfSize(Reachability.Empty, NodesSize);
             Reachability2 = CreateReachabilityOfSize(Reachability1, NodesSize);
             Reachability3 = AddUnreachable(Reachability1, NodesSize / 2);
-            Allowed =  Reachability1.Versions.Keys.ToHashSet();
+            Allowed =  Reachability1.Versions.Keys.ToImmutableHashSet();
         }
 
         private void CheckThunkFor(Reachability r1, Reachability r2, Action<Reachability, Reachability> thunk,
diff --git a/src/core/Akka.Cluster/Gossip.cs b/src/core/Akka.Cluster/Gossip.cs
index 3777459fd95..0a680a4d939 100644
--- a/src/core/Akka.Cluster/Gossip.cs
+++ b/src/core/Akka.Cluster/Gossip.cs
@@ -282,7 +282,7 @@ public Gossip Merge(Gossip that)
             var mergedMembers = EmptyMembers.Union(Member.PickHighestPriority(this._members, that._members));
 
             // 3. merge reachability table by picking records with highest version
-            var mergedReachability = _overview.Reachability.Merge(mergedMembers.Select(m => m.UniqueAddress),
+            var mergedReachability = _overview.Reachability.Merge(mergedMembers.Select(m => m.UniqueAddress).ToImmutableSortedSet(),
                 that._overview.Reachability);
 
             // 4. Nobody can have seen this new gossip yet
diff --git a/src/core/Akka.Cluster/Reachability.cs b/src/core/Akka.Cluster/Reachability.cs
index c7da1f16412..133f36a159a 100644
--- a/src/core/Akka.Cluster/Reachability.cs
+++ b/src/core/Akka.Cluster/Reachability.cs
@@ -55,7 +55,6 @@ public enum ReachabilityStatus
         public static readonly Reachability Empty =
             new Reachability(ImmutableList.Create<Record>(), ImmutableDictionary.Create<UniqueAddress, long>());
 
-        //TODO: Serialization should ignore
         private readonly Lazy<Cache> _cache;
 
         /// <summary>
@@ -80,11 +79,6 @@ public Reachability(ImmutableList<Record> records, ImmutableDictionary<UniqueAdd
         /// </summary>
         public ImmutableDictionary<UniqueAddress, long> Versions { get; }
 
-        /*
-         *  def isReachable(observer: UniqueAddress, subject: UniqueAddress): Boolean =
-            status(observer, subject) == Reachable
-         */
-
         /// <summary>
         ///     TBD
         /// </summary>
@@ -178,7 +172,11 @@ private Reachability Change(UniqueAddress observer, UniqueAddress subject, Reach
             var newVersions = Versions.SetItem(observer, v);
             var newRecord = new Record(observer, subject, status, v);
             var oldObserverRows = ObserverRows(observer);
+
+            // don't record Reachable observation if nothing has been noted so far
             if (oldObserverRows == null && status == ReachabilityStatus.Reachable) return this;
+
+            // otherwise, create new instance including this first observation
             if (oldObserverRows == null) return new Reachability(Records.Add(newRecord), newVersions);
 
             if (!oldObserverRows.TryGetValue(subject, out var oldRecord))
@@ -206,7 +204,7 @@ private Reachability Change(UniqueAddress observer, UniqueAddress subject, Reach
         /// <param name="allowed">TBD</param>
         /// <param name="other">TBD</param>
         /// <returns>TBD</returns>
-        public Reachability Merge(IEnumerable<UniqueAddress> allowed, Reachability other)
+        public Reachability Merge(IImmutableSet<UniqueAddress> allowed, Reachability other)
         {
             var recordBuilder = ImmutableList.CreateBuilder<Record>();
             //TODO: Size hint somehow?
@@ -337,7 +335,7 @@ public bool IsReachable(UniqueAddress observer, UniqueAddress subject)
         public ImmutableHashSet<UniqueAddress> AllUnreachableFrom(UniqueAddress observer)
         {
             var observerRows = ObserverRows(observer);
-            if (observerRows == null) return ImmutableHashSet.Create<UniqueAddress>();
+            if (observerRows == null) return ImmutableHashSet<UniqueAddress>.Empty;
             return
                 ImmutableHashSet.CreateRange(
                     observerRows.Where(p => p.Value.Status == ReachabilityStatus.Unreachable).Select(p => p.Key));
@@ -351,16 +349,18 @@ public ImmutableHashSet<UniqueAddress> AllUnreachableFrom(UniqueAddress observer
         public ImmutableList<Record> RecordsFrom(UniqueAddress observer)
         {
             var rows = ObserverRows(observer);
-            if (rows == null) return ImmutableList.Create<Record>();
+            if (rows == null) return ImmutableList<Record>.Empty;
             return rows.Values.ToImmutableList();
         }
 
+        /// only used for testing
         /// <inheritdoc />
         public override int GetHashCode()
         {
             return Versions.GetHashCode();
         }
 
+        /// only used for testing
         /// <inheritdoc />
         public override bool Equals(object obj)
         {
@@ -467,10 +467,10 @@ public Cache(ImmutableList<Record> records)
             {
                 if (records.IsEmpty)
                 {
-                    ObserverRowMap = ImmutableDictionary
-                        .Create<UniqueAddress, ImmutableDictionary<UniqueAddress, Record>>();
-                    AllTerminated = ImmutableHashSet.Create<UniqueAddress>();
-                    AllUnreachable = ImmutableHashSet.Create<UniqueAddress>();
+                    ObserverRowMap = ImmutableDictionary<UniqueAddress, ImmutableDictionary<UniqueAddress, Record>>
+                        .Empty;
+                    AllTerminated = ImmutableHashSet<UniqueAddress>.Empty;
+                    AllUnreachable = ImmutableHashSet<UniqueAddress>.Empty;
                 }
                 else
                 {
@@ -480,16 +480,12 @@ public Cache(ImmutableList<Record> records)
 
                     foreach (var r in records)
                     {
-                        ImmutableDictionary<UniqueAddress, Record> m = mapBuilder.TryGetValue(r.Observer, out m)
-                            ? m.SetItem(r.Subject, r)
-                            //TODO: Other collections take items for Create. Create unnecessary array here
-                            : ImmutableDictionary.CreateRange(new[]
-                            {
-                                new KeyValuePair<UniqueAddress, Record>(r.Subject, r)
-                            });
+                        ImmutableDictionary<UniqueAddress, Record> m = mapBuilder.TryGetValue(r.Observer, out var mR)
+                            ? mR.SetItem(r.Subject, r)
+                            : ImmutableDictionary<UniqueAddress, Record>.Empty.Add(r.Subject, r);
 
 
-                        mapBuilder.AddOrSet(r.Observer, m);
+                        mapBuilder[r.Observer] = m;
 
                         if (r.Status == ReachabilityStatus.Unreachable) unreachableBuilder.Add(r.Subject);
                         else if (r.Status == ReachabilityStatus.Terminated) terminatedBuilder.Add(r.Subject);
@@ -514,12 +510,12 @@ public ImmutableDictionary<UniqueAddress, ImmutableDictionary<UniqueAddress, Rec
             }
 
             /// <summary>
-            ///     TBD
+            /// Contains all nodes that have been observed as Terminated by at least one other node.
             /// </summary>
             public ImmutableHashSet<UniqueAddress> AllTerminated { get; }
 
             /// <summary>
-            ///     TBD
+            ///  Contains all nodes that have been observed as Unreachable by at least one other node.
             /// </summary>
             public ImmutableHashSet<UniqueAddress> AllUnreachable { get; }