diff --git a/src/main/kotlin/org/phoenixframework/Socket.kt b/src/main/kotlin/org/phoenixframework/Socket.kt
index 4d3ddef..a954121 100644
--- a/src/main/kotlin/org/phoenixframework/Socket.kt
+++ b/src/main/kotlin/org/phoenixframework/Socket.kt
@@ -199,6 +199,7 @@ class Socket(
    * that contain the ref of the message to send and the callback that will send the message.
    */
   internal var sendBuffer: MutableList<Pair<String?, () -> Unit>> = ArrayList()
+  private val sendBufferLock = Any()
 
   /** Ref counter for messages */
   internal var ref: Int = 0
@@ -415,7 +416,9 @@ class Socket(
     } else {
       // If the socket is not connected, add the push to a buffer which will
       // be sent immediately upon connection.
-      sendBuffer.add(Pair(ref, callback))
+      synchronized(sendBufferLock) {
+        sendBuffer.add(Pair(ref, callback))
+      }
     }
   }
 
@@ -464,17 +467,21 @@ class Socket(
 
   /** Send all messages that were buffered before the socket opened */
   internal fun flushSendBuffer() {
-    if (isConnected && sendBuffer.isNotEmpty()) {
-      this.sendBuffer.forEach { it.second.invoke() }
-      this.sendBuffer.clear()
+    synchronized(sendBufferLock) {
+      if (isConnected && sendBuffer.isNotEmpty()) {
+        this.sendBuffer.forEach { it.second.invoke() }
+        this.sendBuffer.clear()
+      }
     }
   }
 
   /** Removes an item from the send buffer with the matching ref */
   internal fun removeFromSendBuffer(ref: String) {
-    this.sendBuffer = this.sendBuffer
-      .filter { it.first != ref }
-      .toMutableList()
+    synchronized(sendBufferLock) {
+      this.sendBuffer = this.sendBuffer
+        .filter { it.first != ref }
+        .toMutableList()
+    }
   }
 
   internal fun leaveOpenTopic(topic: String) {