diff --git a/src/AsyncTCP.cpp b/src/AsyncTCP.cpp
index e0f301c..5871d68 100644
--- a/src/AsyncTCP.cpp
+++ b/src/AsyncTCP.cpp
@@ -271,6 +271,7 @@ AsyncClient::AsyncClient(int sockfd)
 , _writeSpaceRemaining(TCP_SND_BUF)
 , _conn_state(0)
 {
+    _write_mutex = xSemaphoreCreateMutex();
     if (sockfd != -1) {
         int r = fcntl( sockfd, F_SETFL, fcntl( sockfd, F_GETFL, 0 ) | O_NONBLOCK );
 
@@ -564,25 +565,23 @@ bool AsyncClient::_sockIsWriteable(void)
     bool activity = false;
     bool hasErr = false;
 
-    //Serial.print("AsyncClient::_sockIsWriteable: "); Serial.println(_socket);
+    int sent_errno = 0;
+    size_t sent_cb_length = 0;
+    uint32_t sent_cb_delay = 0;
 
     // Socket is now writeable. What should we do?
     switch (_conn_state) {
     case 2:
     case 3:
-        //Serial.println("\tconnect end");
         // Socket has finished connecting. What happened?
         len = (socklen_t)sizeof(int);
         res = getsockopt(_socket, SOL_SOCKET, SO_ERROR, &sockerr, &len);
         if (res < 0) {
-            //Serial.printf("\terrno=%d\r\n", errno);
             _error(errno);
         } else if (sockerr != 0) {
-            //Serial.printf("\tsockerr=%d\r\n", errno);
             _error(sockerr);
         } else {
             // Socket is now fully connected
-            //Serial.println("SUCCESS");
             _conn_state = 4;
             activity = true;
             _rx_last_packet = millis();
@@ -596,29 +595,23 @@ bool AsyncClient::_sockIsWriteable(void)
     case 4:
     default:
         // Socket can accept some new data...
-        //Serial.printf("\tbefore: remaining %d\r\n", _writeSpaceRemaining);
+        xSemaphoreTake(_write_mutex, (TickType_t)portMAX_DELAY);
         if (_writeQueue.size() > 0) {
-            //Serial.printf("\tbuffers remaining: %d\r\n", _writeQueue.size());
             if (_writeQueue.front().written < _writeQueue.front().length) {
                 uint8_t * p = _writeQueue.front().data + _writeQueue.front().written;
                 size_t n = _writeQueue.front().length - _writeQueue.front().written;
-                //Serial.printf("\tlwip_write(%p, %d) ... ", p, n);
                 errno = 0; ssize_t r = lwip_write(_socket, p, n);
-                //Serial.printf("r=%d errno=%d\r\n", r, errno);
 
                 if (r >= 0) {
                     // Written some data into the socket
                     _writeQueue.front().written += r;
                     _writeSpaceRemaining += r;
                     activity = true;
-                    //Serial.printf("\tduring: remaining %d\r\n", _writeSpaceRemaining);
                 } else if (errno == EAGAIN || errno == EWOULDBLOCK) {
-                    // Socket is full again
-                    //Serial.println("\tEAGAIN");
-                    break;  // NOTE: breaks from switch()
+                    // Socket is full, could not write anything
                 } else {
                     hasErr = true;
-                    _error(errno);
+                    sent_errno = errno;
                 }
             }
 
@@ -626,14 +619,18 @@ bool AsyncClient::_sockIsWriteable(void)
                 // Buffer has been fully written to the socket
                 _rx_last_packet = millis();
                 if (_writeQueue.front().owned) ::free(_writeQueue.front().data);
-                if (_sent_cb) {
-                    _sent_cb(_sent_cb_arg, this, _writeQueue.front().length, (millis() - _writeQueue.front().queued_at));
-                }
-
+                sent_cb_length = _writeQueue.front().length;
+                uint32_t sent_cb_delay = millis() - _writeQueue.front().queued_at;
                 _writeQueue.pop_front();
             }
         }
-        //Serial.printf("\tafter: remaining %d\r\n", _writeSpaceRemaining);
+        xSemaphoreGive(_write_mutex);
+
+        if (hasErr) {
+            _error(sent_errno);
+        } else if (sent_cb_length > 0 && _sent_cb) {
+            _sent_cb(_sent_cb_arg, this, sent_cb_length, sent_cb_delay);
+        }
 
         break;
     }
@@ -643,12 +640,8 @@ bool AsyncClient::_sockIsWriteable(void)
 
 void AsyncClient::_sockIsReadable(void)
 {
-    //Serial.print("AsyncClient::_sockIsReadable: "); Serial.println(_socket);
-
     _rx_last_packet = millis();
-    //Serial.print("\tlwip_read ... ");
     errno = 0; ssize_t r = lwip_read(_socket, _readBuffer, MAX_PAYLOAD_SIZE);
-    //Serial.printf("r=%d errno=%d\r\n", r, errno);
     if (r > 0) {
         if(_recv_cb) {
             _recv_cb(_recv_cb_arg, this, _readBuffer, r);
@@ -658,7 +651,7 @@ void AsyncClient::_sockIsReadable(void)
         _close();
     } else if (r < 0) {
         if (errno == EAGAIN || errno == EWOULDBLOCK) {
-            //Serial.println("\tEAGAIN");
+            // Do nothing, will try later
         } else {
             _error(errno);
         }
@@ -671,22 +664,22 @@ void AsyncClient::_sockPoll(void)
 
     uint32_t now = millis();
 
-    //Serial.print("AsyncClient::_sockPoll: "); Serial.println(_socket);
-
     // ACK Timeout - simulated by write queue staleness
-    if (_writeQueue.size() > 0 && !_ack_timeout_signaled && _ack_timeout && (now - _writeQueue.front().queued_at) >= _ack_timeout) {
+    xSemaphoreTake(_write_mutex, (TickType_t)portMAX_DELAY);
+    uint32_t sent_delay = now - _writeQueue.front().queued_at;
+    if (_writeQueue.size() > 0 && !_ack_timeout_signaled && _ack_timeout && sent_delay >= _ack_timeout) {
         _ack_timeout_signaled = true;
         //log_w("ack timeout %d", pcb->state);
-        //Serial.println("\tACK TIMEOUT");
+        xSemaphoreGive(_write_mutex);
         if(_timeout_cb)
-            _timeout_cb(_timeout_cb_arg, this, (now - _writeQueue.front().queued_at));
+            _timeout_cb(_timeout_cb_arg, this, sent_delay);
         return;
     }
+    xSemaphoreGive(_write_mutex);
 
     // RX Timeout
     if (_rx_since_timeout && (now - _rx_last_packet) >= (_rx_since_timeout * 1000)) {
         //log_w("rx timeout %d", pcb->state);
-        //Serial.println("\tRX TIMEOUT");
         _close();
         return;
     }
@@ -753,9 +746,12 @@ size_t AsyncClient::add(const char* data, size_t size, uint8_t apiflags)
     n_entry.written = 0;
     n_entry.queued_at = millis();
 
+    xSemaphoreTake(_write_mutex, (TickType_t)portMAX_DELAY);
     _writeQueue.push_back(n_entry);
     _writeSpaceRemaining -= will_send;
     _ack_timeout_signaled = false;
+    xSemaphoreGive(_write_mutex);
+
     return will_send;
 }
 
diff --git a/src/AsyncTCP.h b/src/AsyncTCP.h
index 6ac9601..b6691db 100644
--- a/src/AsyncTCP.h
+++ b/src/AsyncTCP.h
@@ -180,6 +180,7 @@ class AsyncClient : public AsyncSocketBase
     } queued_writebuf;
 
     // Queue of buffers to write to socket
+    SemaphoreHandle_t _write_mutex;
     std::deque<queued_writebuf> _writeQueue;
     bool _ack_timeout_signaled = false;