Index: core/src/main/java/quickfix/Session.java
===================================================================
--- core/src/main/java/quickfix/Session.java	(revision 926)
+++ core/src/main/java/quickfix/Session.java	(working copy)
@@ -27,6 +27,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.locks.ReentrantLock;
 
 import quickfix.Message.Header;
 import quickfix.field.BeginSeqNo;
@@ -231,7 +232,7 @@
 
     private boolean enabled;
 
-    private final String responderSync = "SessionResponderSync";
+    private final ReentrantLock responderSync = new ReentrantLock(true);
     // @GuardedBy(responderSync)
     private Responder responder;
 
@@ -330,19 +331,25 @@
      * @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();
         }
     }
 
@@ -1436,13 +1443,16 @@
      * @throws IOException IO error
      */
     public void disconnect() throws IOException {
-        synchronized (responderSync) {
+        responderSync.lock();
+        try {
             if (!hasResponder()) {
                 return;
             }
             getLog().onEvent("Disconnecting");
             responder.disconnect();
             setResponder(null);
+        } finally {
+            responderSync.unlock();
         }
         
         boolean logonReceived = state.isLogonReceived();
@@ -1778,14 +1788,19 @@
 
     private boolean send(String messageString) {
         getLog().onOutgoing(messageString);
-        synchronized (responderSync) {
-            if (!hasResponder()) {
-                getLog().onEvent(
-                        "Attempt to send while not connected (message stored until connected).");
-                return false;
-            }
-            return getResponder().send(messageString);
+        Responder responder;
+        responderSync.lock();
+        try {
+            responder = this.responder;
+        } finally {
+            responderSync.unlock();
         }
+        if (responder == null) {
+            getLog().onEvent(
+                    "Attempt to send while not connected (message stored until connected).");
+            return false;
+        }
+        return responder.send(messageString);
     }
 
     private boolean isCorrectCompID(String senderCompID, String targetCompID) {