diff --git a/src/Orchard.Web/Modules/Orchard.OutputCache/Services/CacheService.cs b/src/Orchard.Web/Modules/Orchard.OutputCache/Services/CacheService.cs
index 5aee5d092..2be361bf9 100644
--- a/src/Orchard.Web/Modules/Orchard.OutputCache/Services/CacheService.cs
+++ b/src/Orchard.Web/Modules/Orchard.OutputCache/Services/CacheService.cs
@@ -37,11 +37,8 @@ namespace Orchard.OutputCache.Services {
         }
 
         public void RemoveByTag(string tag) {
-            Collection<string> itemKeys;
-            if (_tagCache.TryGetValue(tag, out itemKeys)) {
-                foreach (var key in itemKeys) {
-                    _cacheStorageProvider.Remove(key);
-                }
+            foreach(var key in _tagCache.GetTaggedItems(tag)) {
+                _cacheStorageProvider.Remove(key);
             }
         }
 
diff --git a/src/Orchard.Web/Modules/Orchard.OutputCache/Services/DefaultTagCache.cs b/src/Orchard.Web/Modules/Orchard.OutputCache/Services/DefaultTagCache.cs
index 7abaee684..45a4a9d71 100644
--- a/src/Orchard.Web/Modules/Orchard.OutputCache/Services/DefaultTagCache.cs
+++ b/src/Orchard.Web/Modules/Orchard.OutputCache/Services/DefaultTagCache.cs
@@ -1,24 +1,41 @@
 using System;
 using System.Collections.Concurrent;
-using System.Collections.ObjectModel;
+using System.Collections.Generic;
+using System.Linq;
 using Orchard.OutputCache.Models;
+using Orchard.Utility.Extensions;
 
 namespace Orchard.OutputCache.Services {
     /// <summary>
     /// Tenant wide case insensitive reverse index for <see cref="CacheItem"/> tags.
     /// </summary>
-    public class DefaultTagCache : ConcurrentDictionary<string, Collection<string>>, ITagCache {
-        public DefaultTagCache() : base(StringComparer.OrdinalIgnoreCase) {
+    public class DefaultTagCache : ITagCache {
+
+        private readonly ConcurrentDictionary<string, HashSet<string>> _dictionary;
+
+        public DefaultTagCache() {
+            _dictionary = new ConcurrentDictionary<string, HashSet<string>>(StringComparer.OrdinalIgnoreCase);
         }
 
         public void Tag(string tag, params string[] keys) {
-            var collection = GetOrAdd(tag, x => new Collection<string>());
+            var set = _dictionary.GetOrAdd(tag, x => new HashSet<string>());
 
-            foreach (var key in keys) {
-                if (!collection.Contains(key)) {
-                    collection.Add(key);
+            lock (set) {
+                foreach (var key in keys) {
+                    set.Add(key);
                 }
             }
         }
+        
+        public IEnumerable<string> GetTaggedItems(string tag) {
+            HashSet<string> set;
+            if (_dictionary.TryGetValue(tag, out set)) {
+                lock (set) {
+                    return set.ToReadOnlyCollection();
+                }
+            }
+
+            return Enumerable.Empty<string>();
+        }
     }
 }
\ No newline at end of file
diff --git a/src/Orchard.Web/Modules/Orchard.OutputCache/Services/ITagCache.cs b/src/Orchard.Web/Modules/Orchard.OutputCache/Services/ITagCache.cs
index 916c136e1..a0fcc46d0 100644
--- a/src/Orchard.Web/Modules/Orchard.OutputCache/Services/ITagCache.cs
+++ b/src/Orchard.Web/Modules/Orchard.OutputCache/Services/ITagCache.cs
@@ -1,9 +1,8 @@
 using System.Collections.Generic;
-using System.Collections.ObjectModel;
-using Orchard;
 
 namespace Orchard.OutputCache.Services {
-    public interface ITagCache : IDictionary<string, Collection<string>>, ISingletonDependency {
+    public interface ITagCache : ISingletonDependency {
         void Tag(string tag, params string[] keys);
+        IEnumerable<string> GetTaggedItems(string tag);
     }
 }
\ No newline at end of file