diff --git a/OpenRa.FileFormats/FileSystem.cs b/OpenRa.FileFormats/FileSystem.cs
index d8e1259159..686f699c4a 100644
--- a/OpenRa.FileFormats/FileSystem.cs
+++ b/OpenRa.FileFormats/FileSystem.cs
@@ -1,11 +1,13 @@
 using System.Collections.Generic;
 using System.IO;
+using System.Linq;
 
 namespace OpenRa.FileFormats
 {
 	public static class FileSystem
 	{
 		static List<IFolder> mountedFolders = new List<IFolder>();
+		static List<IFolder> temporaryMounts = new List<IFolder>();
 
 		public static void MountDefault( bool useAftermath )
 		{
@@ -33,6 +35,18 @@ namespace OpenRa.FileFormats
 			mountedFolders.Add(folder);
 		}
 
+		public static void MountTemporary(IFolder folder)
+		{
+			mountedFolders.Add(folder);
+			temporaryMounts.Add(folder);
+		}
+
+		public static void UnmountTemporaryPackages()
+		{
+			mountedFolders.RemoveAll(f => temporaryMounts.Contains(f));
+			temporaryMounts.Clear();
+		}
+
 		public static Stream Open(string filename)
 		{
 			foreach( IFolder folder in mountedFolders )
diff --git a/OpenRa.Game/Game.cs b/OpenRa.Game/Game.cs
index c8aa8ed418..425d65c1f8 100644
--- a/OpenRa.Game/Game.cs
+++ b/OpenRa.Game/Game.cs
@@ -56,7 +56,9 @@ namespace OpenRa.Game
 		public static void ChangeMap(string mapName)
 		{
 			SheetBuilder.Initialize(renderer);
-			
+			SpriteSheetBuilder.Initialize();
+			UnitSheetBuilder.Initialize();
+			FileSystem.UnmountTemporaryPackages();
 			Rules.LoadRules(mapName, usingAftermath);
 			palette = new HardwarePalette(renderer, Rules.Map);
 
diff --git a/OpenRa.Game/GameRules/Rules.cs b/OpenRa.Game/GameRules/Rules.cs
index aceda21226..25be1f7693 100755
--- a/OpenRa.Game/GameRules/Rules.cs
+++ b/OpenRa.Game/GameRules/Rules.cs
@@ -89,7 +89,7 @@ namespace OpenRa.Game
 
 			TechTree = new TechTree();
 			Map = new Map( AllRules );
-			FileSystem.Mount( new Package( Rules.Map.Theater + ".mix" ) );
+			FileSystem.MountTemporary( new Package( Rules.Map.Theater + ".mix" ) );
 			TileSet = new TileSet( Map.TileSuffix );
 		}
 
diff --git a/OpenRa.Game/Graphics/SpriteSheetBuilder.cs b/OpenRa.Game/Graphics/SpriteSheetBuilder.cs
index 200ab655f1..beb57caea0 100644
--- a/OpenRa.Game/Graphics/SpriteSheetBuilder.cs
+++ b/OpenRa.Game/Graphics/SpriteSheetBuilder.cs
@@ -5,8 +5,12 @@ namespace OpenRa.Game.Graphics
 {
 	static class SpriteSheetBuilder
 	{
-		static Dictionary<string, Sprite[]> sprites =
-			new Dictionary<string, Sprite[]>();
+		public static void Initialize()
+		{
+			sprites = new Dictionary<string, Sprite[]>();
+		}
+
+		static Dictionary<string, Sprite[]> sprites;
 
 		public static Sprite LoadSprite(string filename, params string[] exts )
 		{
diff --git a/OpenRa.Game/Graphics/UnitSheetBuilder.cs b/OpenRa.Game/Graphics/UnitSheetBuilder.cs
index 6e423517c7..fba8d61661 100644
--- a/OpenRa.Game/Graphics/UnitSheetBuilder.cs
+++ b/OpenRa.Game/Graphics/UnitSheetBuilder.cs
@@ -6,8 +6,14 @@ namespace OpenRa.Game.Graphics
 {
 	static class UnitSheetBuilder
 	{
-		public static readonly List<Sprite> sprites = new List<Sprite>();
-		static Dictionary<string, Range<int>> sequences = new Dictionary<string, Range<int>>();
+		public static void Initialize()
+		{
+			sprites = new List<Sprite>();
+			sequences = new Dictionary<string, Range<int>>();
+		}
+
+		public static List<Sprite> sprites;
+		static Dictionary<string, Range<int>> sequences;
 
 		public static Range<int> GetUnit(string name)
 		{