diff --git a/fml/build.xml b/fml/build.xml
index ac42f2c663..e08af069d5 100644
--- a/fml/build.xml
+++ b/fml/build.xml
@@ -16,6 +16,7 @@
     </exec>
     <propertyfile file="fmlversion.properties">
       <entry key="fmlbuild.build.number" type="int" value="${version.build}"/>
+      <entry key="fmlbuild.deobfuscation.hash" type="string" value="${deobf.checksum}"/>
     </propertyfile>
   </target>
 
@@ -210,6 +211,8 @@
         <globmapper from="packaged.srg" to="joined.srg"/>
       </mappedresources>
     </zip>
+    <checksum algorithm="SHA1" property="deobf.checksum" file="deobfuscation_data_${version.minecraft}.zip"/>
+    <antcall target="writeversion"/>
   </target>
 
   <target name="build" depends="buildenvsetup,merge-client,merge-common,build-universal,build-source-pack,build-deobf-data" />
diff --git a/fml/common/cpw/mods/fml/common/asm/FMLSanityChecker.java b/fml/common/cpw/mods/fml/common/asm/FMLSanityChecker.java
index e4a59b17b4..a2bdd67ed2 100644
--- a/fml/common/cpw/mods/fml/common/asm/FMLSanityChecker.java
+++ b/fml/common/cpw/mods/fml/common/asm/FMLSanityChecker.java
@@ -126,7 +126,7 @@ public class FMLSanityChecker implements IFMLCallHook
     public void injectData(Map<String, Object> data)
     {
         cl = (RelaunchClassLoader) data.get("classLoader");
-        FMLDeobfuscatingRemapper.INSTANCE.setup((File)data.get("mcLocation"), cl);
+        FMLDeobfuscatingRemapper.INSTANCE.setup((File)data.get("mcLocation"), cl, (String) data.get("deobfuscationFileName"));
     }
 
 }
diff --git a/fml/common/cpw/mods/fml/common/asm/transformers/deobf/FMLDeobfuscatingRemapper.java b/fml/common/cpw/mods/fml/common/asm/transformers/deobf/FMLDeobfuscatingRemapper.java
index 68bb2bf2d0..7107876797 100644
--- a/fml/common/cpw/mods/fml/common/asm/transformers/deobf/FMLDeobfuscatingRemapper.java
+++ b/fml/common/cpw/mods/fml/common/asm/transformers/deobf/FMLDeobfuscatingRemapper.java
@@ -51,15 +51,16 @@ public class FMLDeobfuscatingRemapper extends Remapper {
 
     private FMLDeobfuscatingRemapper()
     {
+        classNameBiMap=ImmutableBiMap.of();
     }
 
-    public void setup(File mcDir, RelaunchClassLoader classLoader)
+    public void setup(File mcDir, RelaunchClassLoader classLoader, String deobfFileName)
     {
         this.classLoader = classLoader;
         try
         {
             File libDir = new File(mcDir, "lib");
-            File mapData = new File(libDir, "deobfuscation_data.zip");
+            File mapData = new File(libDir, deobfFileName);
             ZipFile mapZip = new ZipFile(mapData);
             ZipEntry classData = mapZip.getEntry("joined.srg");
             ZipInputSupplier zis = new ZipInputSupplier(mapZip, classData);
diff --git a/fml/common/cpw/mods/fml/relauncher/CoreFMLLibraries.java b/fml/common/cpw/mods/fml/relauncher/CoreFMLLibraries.java
index 400ae5b465..f74bc9cdea 100644
--- a/fml/common/cpw/mods/fml/relauncher/CoreFMLLibraries.java
+++ b/fml/common/cpw/mods/fml/relauncher/CoreFMLLibraries.java
@@ -2,8 +2,8 @@ package cpw.mods.fml.relauncher;
 
 public class CoreFMLLibraries implements ILibrarySet
 {
-    private static String[] libraries = { "argo-small-3.2.jar","guava-14.0-rc3.jar","asm-all-4.1.jar", "bcprov-jdk15on-148.jar" };
-    private static String[] checksums = { "58912ea2858d168c50781f956fa5b59f0f7c6b51", "931ae21fa8014c3ce686aaa621eae565fefb1a6a", "054986e962b88d8660ae4566475658469595ef58", "960dea7c9181ba0b17e8bab0c06a43f0a5f04e65" };
+    private static String[] libraries = { "argo-small-3.2.jar","guava-14.0-rc3.jar","asm-all-4.1.jar", "bcprov-jdk15on-148.jar", FMLInjectionData.debfuscationDataName() };
+    private static String[] checksums = { "58912ea2858d168c50781f956fa5b59f0f7c6b51", "931ae21fa8014c3ce686aaa621eae565fefb1a6a", "054986e962b88d8660ae4566475658469595ef58", "960dea7c9181ba0b17e8bab0c06a43f0a5f04e65", FMLInjectionData.deobfuscationDataHash };
 
     @Override
     public String[] getLibraries()
diff --git a/fml/common/cpw/mods/fml/relauncher/FMLInjectionData.java b/fml/common/cpw/mods/fml/relauncher/FMLInjectionData.java
index c4b86c6042..3ef47d5b8d 100644
--- a/fml/common/cpw/mods/fml/relauncher/FMLInjectionData.java
+++ b/fml/common/cpw/mods/fml/relauncher/FMLInjectionData.java
@@ -17,6 +17,8 @@ public class FMLInjectionData
     static String build;
     static String mccversion;
     static String mcpversion;
+    static String deobfuscationDataHash;
+
     public static List<String> containers = new ArrayList<String>();
 
     static void build(File mcHome, RelaunchClassLoader classLoader)
@@ -43,10 +45,13 @@ public class FMLInjectionData
         build = properties.getProperty("fmlbuild.build.number", "missing");
         mccversion = properties.getProperty("fmlbuild.mcversion", "missing");
         mcpversion = properties.getProperty("fmlbuild.mcpversion", "missing");
-
-
+        deobfuscationDataHash = properties.getProperty("fmlbuild.deobfuscation.hash","deadbeef");
     }
 
+    static String debfuscationDataName()
+    {
+        return "deobfuscation_data_"+mccversion+".zip";
+    }
     public static Object[] data()
     {
         return new Object[] { major, minor, rev, build, mccversion, mcpversion, minecraftHome, containers };
diff --git a/fml/common/cpw/mods/fml/relauncher/RelaunchLibraryManager.java b/fml/common/cpw/mods/fml/relauncher/RelaunchLibraryManager.java
index 1532af9956..e95e79e21a 100644
--- a/fml/common/cpw/mods/fml/relauncher/RelaunchLibraryManager.java
+++ b/fml/common/cpw/mods/fml/relauncher/RelaunchLibraryManager.java
@@ -35,8 +35,23 @@ public class RelaunchLibraryManager
     private static Map<IFMLLoadingPlugin, File> pluginLocations;
     private static List<IFMLLoadingPlugin> loadPlugins;
     private static List<ILibrarySet> libraries;
+    private static boolean deobfuscatedEnvironment;
+
     public static void handleLaunch(File mcDir, RelaunchClassLoader actualClassLoader)
     {
+        try
+        {
+            // Are we in a 'decompiled' environment?
+            actualClassLoader.getClassBytes("net.minecraft.world.World");
+            FMLRelaunchLog.info("Managed to load a deobfuscated Minecraft name- we are in a deobfuscated environment. Skipping runtime deobfuscation");
+            deobfuscatedEnvironment = true;
+        }
+        catch (IOException e1)
+        {
+            FMLRelaunchLog.fine("Enabling runtime deobfuscation");
+            deobfuscatedEnvironment = false;
+        }
+
         pluginLocations = new HashMap<IFMLLoadingPlugin, File>();
         loadPlugins = new ArrayList<IFMLLoadingPlugin>();
         libraries = new ArrayList<ILibrarySet>();
@@ -240,8 +255,10 @@ public class RelaunchLibraryManager
             }
         }
         // Deobfuscation transformer, always last
-        actualClassLoader.registerTransformer("cpw.mods.fml.common.asm.transformers.DeobfuscationTransformer");
-
+        if (!deobfuscatedEnvironment)
+        {
+            actualClassLoader.registerTransformer("cpw.mods.fml.common.asm.transformers.DeobfuscationTransformer");
+        }
         downloadMonitor.updateProgressString("Running coremod plugins");
         Map<String,Object> data = new HashMap<String,Object>();
         data.put("mcLocation", mcDir);
@@ -259,6 +276,7 @@ public class RelaunchLibraryManager
                     IFMLCallHook call = (IFMLCallHook) Class.forName(setupClass, true, actualClassLoader).newInstance();
                     Map<String,Object> callData = new HashMap<String, Object>();
                     callData.put("classLoader", actualClassLoader);
+                    callData.put("deobfuscationFileName", FMLInjectionData.debfuscationDataName());
                     call.injectData(callData);
                     call.call();
                 }