diff --git a/tools/cooja/apps/serial_socket/java/org/contikios/cooja/serialsocket/SerialSocketClient.java b/tools/cooja/apps/serial_socket/java/org/contikios/cooja/serialsocket/SerialSocketClient.java
index e074437d2..113730952 100644
--- a/tools/cooja/apps/serial_socket/java/org/contikios/cooja/serialsocket/SerialSocketClient.java
+++ b/tools/cooja/apps/serial_socket/java/org/contikios/cooja/serialsocket/SerialSocketClient.java
@@ -115,10 +115,12 @@ public class SerialSocketClient extends VisPlugin implements MotePlugin {
   private DataOutputStream out;
 
   private final Mote mote;
+  private final Simulation simulation;
 
   public SerialSocketClient(Mote mote, Simulation simulation, final Cooja gui) {
     super("Serial Socket (CLIENT) (" + mote + ")", gui, false);
     this.mote = mote;
+    this.simulation = simulation;
 
     /* GUI components */
     if (Cooja.isVisualized()) {
@@ -411,9 +413,19 @@ public class SerialSocketClient extends VisPlugin implements MotePlugin {
           }
 
           if (numRead >= 0) {
-            for (int i = 0; i < numRead; i++) {
-              serialPort.writeByte(data[i]);
-            }
+            final int finalNumRead = numRead;
+            final byte[] finalData = data;
+            /* We are not on the simulation thread */
+            simulation.invokeSimulationThread(new Runnable() {
+
+              @Override
+              public void run() {
+                for (int i = 0; i < finalNumRead; i++) {
+                  serialPort.writeByte(finalData[i]);
+                }
+              }
+            });
+
             inBytes += numRead;
             if (Cooja.isVisualized()) {
               socketToMoteLabel.setText(inBytes + " bytes");
diff --git a/tools/cooja/apps/serial_socket/java/org/contikios/cooja/serialsocket/SerialSocketServer.java b/tools/cooja/apps/serial_socket/java/org/contikios/cooja/serialsocket/SerialSocketServer.java
index e3aabbc15..fa55da74b 100644
--- a/tools/cooja/apps/serial_socket/java/org/contikios/cooja/serialsocket/SerialSocketServer.java
+++ b/tools/cooja/apps/serial_socket/java/org/contikios/cooja/serialsocket/SerialSocketServer.java
@@ -113,10 +113,12 @@ public class SerialSocketServer extends VisPlugin implements MotePlugin {
   private Socket clientSocket;
 
   private Mote mote;
+  private Simulation simulation;
 
   public SerialSocketServer(Mote mote, Simulation simulation, final Cooja gui) {
     super("Serial Socket (SERVER) (" + mote + ")", gui, false);
     this.mote = mote;
+    this.simulation = simulation;
 
     updateTimer.start();
 
@@ -475,10 +477,19 @@ public class SerialSocketServer extends VisPlugin implements MotePlugin {
 
       logger.info("Forwarder: socket -> serial port");
       while (numRead >= 0) {
-        for (int i = 0; i < numRead; i++) {
-          serialPort.writeByte(data[i]);
-        }
-        inBytes += numRead;
+        final int finalNumRead = numRead;
+        final byte[] finalData = data;
+        /* We are not on the simulation thread */
+        simulation.invokeSimulationThread(new Runnable() {
+
+          @Override
+          public void run() {
+            for (int i = 0; i < finalNumRead; i++) {
+              serialPort.writeByte(finalData[i]);
+            }
+            inBytes += finalNumRead;
+          }
+        });
 
         try {
           numRead = in.read(data);