Index: core/src/main/java/quickfix/Session.java
===================================================================
--- core/src/main/java/quickfix/Session.java	(revision 999)
+++ core/src/main/java/quickfix/Session.java	(working copy)
@@ -29,6 +29,7 @@
 import java.util.Map;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
+import java.util.concurrent.locks.ReentrantLock;
 
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -308,7 +309,7 @@
 
     private boolean enabled;
 
-    private final String responderSync = "SessionResponderSync";
+    private final ReentrantLock responderSync = new ReentrantLock(true);
     // @GuardedBy(responderSync)
     private Responder responder;
 
@@ -453,19 +454,25 @@
      * @param responder a responder implementation
      */
     public void setResponder(Responder responder) {
-        synchronized (responderSync) {
+        responderSync.lock();
+        try {
             this.responder = responder;
             if (responder != null) {
                 stateListener.onConnect();
             } else {
                 stateListener.onDisconnect();
             }
+        } finally {
+            responderSync.unlock();
         }
     }
 
     public Responder getResponder() {
-        synchronized (responderSync) {
+        responderSync.lock();
+        try {
             return responder;
+        } finally {
+            responderSync.unlock();
         }
     }
 
@@ -1837,7 +1844,8 @@
      *             IO error
      */
     public void disconnect(String reason, boolean logError) throws IOException {
-        synchronized (responderSync) {
+        responderSync.lock();
+        try {
             if (!hasResponder()) {
                 getLog().onEvent("Already disconnected: " + reason);
                 return;
@@ -1850,6 +1858,8 @@
             }
             responder.disconnect();
             setResponder(null);
+        } finally {
+            responderSync.unlock();
         }
 
         final boolean logonReceived = state.isLogonReceived();
@@ -2215,13 +2225,18 @@
 
     private boolean send(String messageString) {
         getLog().onOutgoing(messageString);
-        synchronized (responderSync) {
-            if (!hasResponder()) {
-                getLog().onEvent("No responder, not sending message: " + messageString);
-                return false;
-            }
-            return getResponder().send(messageString);
+        Responder responder;
+        responderSync.lock();
+        try {
+            responder = this.responder;
+        } finally {
+            responderSync.unlock();
         }
+        if (responder == null) {
+            getLog().onEvent("No responder, not sending message: " + messageString);
+            return false;
+        }
+        return responder.send(messageString);
     }
 
     private boolean isCorrectCompID(String senderCompID, String targetCompID) {