diff --git a/src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/LmxSubtagAlarmSourceTests.cs b/src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/LmxSubtagAlarmSourceTests.cs
index bbfb05d..08995f9 100644
--- a/src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/LmxSubtagAlarmSourceTests.cs
+++ b/src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/LmxSubtagAlarmSourceTests.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using ZB.MOM.WW.MxGateway.Worker.MxAccess;
 
 namespace ZB.MOM.WW.MxGateway.Worker.Tests.MxAccess;
@@ -133,6 +134,60 @@ public sealed class LmxSubtagAlarmSourceTests
         Assert.Single(server.Writes);
     }
 
+    /// <summary>
+    ///     Verifies <see cref="LmxSubtagAlarmSource.Dispose"/> UnAdvises only the
+    ///     handles that were actually advised — a write-only item (added by
+    ///     <see cref="LmxSubtagAlarmSource.Write"/> but never advised) is removed
+    ///     but not unadvised — and unregisters the server exactly once.
+    /// </summary>
+    [Fact]
+    public void Dispose_UnAdvisesOnlyAdvisedHandles_RemovesAll_AndUnregistersOnce()
+    {
+        var server = new RecordingMxAccessServer();
+        var source = new LmxSubtagAlarmSource(server, FakeServerHandle);
+
+        source.Advise(new[] { "Tank1.Alarm.Subtag", "Tank2.Alarm.Subtag" });
+        // A write-only subtag: added by Write, never advised.
+        source.Write("Tank1.Alarm.AckComment", "acknowledged");
+
+        int advised1 = server.LastItemHandleFor("Tank1.Alarm.Subtag");
+        int advised2 = server.LastItemHandleFor("Tank2.Alarm.Subtag");
+        int writeOnly = server.LastItemHandleFor("Tank1.Alarm.AckComment");
+
+        source.Dispose();
+
+        // Only the two advised handles are unadvised — never the write-only one.
+        Assert.Equal(new[] { advised1, advised2 }, server.UnAdvisedItemHandles);
+        Assert.DoesNotContain(writeOnly, server.UnAdvisedItemHandles);
+        // Every added item (advised + write-only) is removed.
+        Assert.Equal(
+            new[] { advised1, advised2, writeOnly }.OrderBy(h => h),
+            server.RemovedItemHandles.OrderBy(h => h));
+        Assert.Equal(1, server.UnregisterCount);
+    }
+
+    /// <summary>
+    ///     Verifies <see cref="LmxSubtagAlarmSource.Dispose"/> is idempotent: a
+    ///     second call performs no further teardown.
+    /// </summary>
+    [Fact]
+    public void Dispose_IsIdempotent()
+    {
+        var server = new RecordingMxAccessServer();
+        var source = new LmxSubtagAlarmSource(server, FakeServerHandle);
+
+        source.Advise(new[] { "Tank1.Alarm.Subtag" });
+
+        source.Dispose();
+        int unadviseAfterFirst = server.UnAdvisedItemHandles.Count;
+        int unregisterAfterFirst = server.UnregisterCount;
+
+        source.Dispose();
+
+        Assert.Equal(unadviseAfterFirst, server.UnAdvisedItemHandles.Count);
+        Assert.Equal(unregisterAfterFirst, server.UnregisterCount);
+    }
+
     /// <summary>
     ///     Recording <see cref="IMxAccessServer"/> test double that captures the
     ///     AddItem/Advise/Write/UnAdvise/RemoveItem/Unregister calls
@@ -152,13 +207,17 @@ public sealed class LmxSubtagAlarmSourceTests
 
         public List<WriteRecord> Writes { get; } = new();
 
+        public List<int> UnAdvisedItemHandles { get; } = new();
+
+        public List<int> RemovedItemHandles { get; } = new();
+
+        public int UnregisterCount { get; private set; }
+
         public int LastItemHandleFor(string itemAddress) => handlesByAddress[itemAddress];
 
         public int Register(string clientName) => FakeServerHandle;
 
-        public void Unregister(int serverHandle)
-        {
-        }
+        public void Unregister(int serverHandle) => UnregisterCount++;
 
         public int AddItem(int serverHandle, string itemDefinition)
         {
@@ -171,9 +230,7 @@ public sealed class LmxSubtagAlarmSourceTests
         public int AddItem2(int serverHandle, string itemDefinition, string itemContext)
             => AddItem(serverHandle, itemDefinition);
 
-        public void RemoveItem(int serverHandle, int itemHandle)
-        {
-        }
+        public void RemoveItem(int serverHandle, int itemHandle) => RemovedItemHandles.Add(itemHandle);
 
         public void Advise(int serverHandle, int itemHandle)
         {
@@ -181,9 +238,7 @@ public sealed class LmxSubtagAlarmSourceTests
             AdvisedServerHandles.Add(serverHandle);
         }
 
-        public void UnAdvise(int serverHandle, int itemHandle)
-        {
-        }
+        public void UnAdvise(int serverHandle, int itemHandle) => UnAdvisedItemHandles.Add(itemHandle);
 
         public void AdviseSupervisory(int serverHandle, int itemHandle)
         {
diff --git a/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/LmxSubtagAlarmSource.cs b/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/LmxSubtagAlarmSource.cs
index c16e2e6..e334a9a 100644
--- a/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/LmxSubtagAlarmSource.cs
+++ b/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/LmxSubtagAlarmSource.cs
@@ -46,6 +46,12 @@ public sealed class LmxSubtagAlarmSource : ISubtagAlarmSource
     private readonly Dictionary<int, string> addressesByItemHandle =
         new Dictionary<int, string>();
 
+    // Handles that were actually Advise()d, tracked separately from the added
+    // set so Dispose only UnAdvises advised items. Write() can AddItem a
+    // write-only subtag (e.g. an ack-comment that was never advised); calling
+    // UnAdvise on such a handle would be an unbalanced teardown.
+    private readonly HashSet<int> advisedItemHandles = new HashSet<int>();
+
     private object? mxAccessComObject;
     private IMxAccessServer? server;
     private LMXProxyServerClass? comEventSource;
@@ -134,6 +140,7 @@ public sealed class LmxSubtagAlarmSource : ISubtagAlarmSource
             mxServer.Advise(serverHandle, itemHandle);
             itemHandlesByAddress[itemAddress!] = itemHandle;
             addressesByItemHandle[itemHandle] = itemAddress!;
+            advisedItemHandles.Add(itemHandle);
         }
     }
 
@@ -225,7 +232,13 @@ public sealed class LmxSubtagAlarmSource : ISubtagAlarmSource
         {
             foreach (KeyValuePair<int, string> entry in addressesByItemHandle)
             {
-                try { mxServer.UnAdvise(serverHandle, entry.Key); } catch { /* swallow — best effort */ }
+                // Only UnAdvise handles that were actually advised; a write-only
+                // item (added by Write but never Advise'd) was never advised.
+                if (advisedItemHandles.Contains(entry.Key))
+                {
+                    try { mxServer.UnAdvise(serverHandle, entry.Key); } catch { /* swallow — best effort */ }
+                }
+
                 try { mxServer.RemoveItem(serverHandle, entry.Key); } catch { /* swallow — best effort */ }
             }
 
@@ -234,6 +247,7 @@ public sealed class LmxSubtagAlarmSource : ISubtagAlarmSource
 
         itemHandlesByAddress.Clear();
         addressesByItemHandle.Clear();
+        advisedItemHandles.Clear();
 
         object? comToRelease = mxAccessComObject;
         mxAccessComObject = null;