diff --git a/esp-link/cgioptiboot.c b/esp-link/cgioptiboot.c
index ed87d0e..b75f4de 100644
--- a/esp-link/cgioptiboot.c
+++ b/esp-link/cgioptiboot.c
@@ -8,6 +8,7 @@
 #include "uart.h"
 #include "stk500.h"
 #include "serbridge.h"
+#include "mqtt_cmd.h"
 #include "serled.h"
 
 #define INIT_DELAY     150   // wait this many millisecs before sending anything
@@ -76,6 +77,7 @@ static void ICACHE_FLASH_ATTR optibootInit() {
   progState = stateInit;
   baudCnt = 0;
   uart0_baud(flashConfig.baud_rate);
+  mqtt_unblock();
   ackWait = 0;
   errMessage[0] = 0;
   responseLen = 0;
@@ -128,6 +130,7 @@ int ICACHE_FLASH_ATTR cgiOptibootSync(HttpdConnData *connData) {
   } else if (connData->requestType == HTTPD_METHOD_POST) {
     // issue reset
     optibootInit();
+    mqtt_block(); // prevent MQTT from interfering
     baudRate = flashConfig.baud_rate;
     programmingCB = optibootUartRecv;
     initBaud();
diff --git a/mqtt/mqtt_cmd.c b/mqtt/mqtt_cmd.c
index 0f40b79..b6bb909 100644
--- a/mqtt/mqtt_cmd.c
+++ b/mqtt/mqtt_cmd.c
@@ -13,8 +13,16 @@
 #define DBG(format, ...) do { } while(0)
 #endif
 
+static bool blocked; // flag to prevent MQTT from sending on serial while trying to PGM uC
+
+void ICACHE_FLASH_ATTR
+mqtt_block() { blocked = true; }
+void ICACHE_FLASH_ATTR
+mqtt_unblock() { blocked = false; }
+
 void ICACHE_FLASH_ATTR
 cmdMqttConnectedCb(MQTT_Client* client) {
+  if (blocked) return;
   MqttCmdCb* cb = (MqttCmdCb*)client->user_data;
   DBG("MQTT: Connected Cb=%p\n", (void*)cb->connectedCb);
   cmdResponseStart(CMD_RESP_CB, cb->connectedCb, 0);
@@ -23,6 +31,7 @@ cmdMqttConnectedCb(MQTT_Client* client) {
 
 void ICACHE_FLASH_ATTR
 cmdMqttDisconnectedCb(MQTT_Client* client) {
+  if (blocked) return;
   MqttCmdCb* cb = (MqttCmdCb*)client->user_data;
   DBG("MQTT: Disconnected cb=%p\n", (void*)cb->disconnectedCb);
   cmdResponseStart(CMD_RESP_CB, cb->disconnectedCb, 0);
@@ -31,6 +40,7 @@ cmdMqttDisconnectedCb(MQTT_Client* client) {
 
 void ICACHE_FLASH_ATTR
 cmdMqttPublishedCb(MQTT_Client* client) {
+  if (blocked) return;
   MqttCmdCb* cb = (MqttCmdCb*)client->user_data;
   DBG("MQTT: Published cb=%p\n", (void*)cb->publishedCb);
   cmdResponseStart(CMD_RESP_CB, cb->publishedCb, 0);
@@ -41,6 +51,7 @@ void ICACHE_FLASH_ATTR
 cmdMqttDataCb(MQTT_Client* client, const char* topic, uint32_t topic_len,
     const char* data, uint32_t data_len)
 {
+  if (blocked) return;
   MqttCmdCb* cb = (MqttCmdCb*)client->user_data;
   DBG("MQTT: Data cb=%p topic=%s len=%u\n", (void*)cb->dataCb, topic, data_len);
 
diff --git a/mqtt/mqtt_cmd.h b/mqtt/mqtt_cmd.h
index 6a14a63..e7121c7 100644
--- a/mqtt/mqtt_cmd.h
+++ b/mqtt/mqtt_cmd.h
@@ -17,4 +17,7 @@ void MQTTCMD_Publish(CmdPacket *cmd);
 void MQTTCMD_Subscribe(CmdPacket *cmd);
 void MQTTCMD_Lwt(CmdPacket *cmd);
 
+void mqtt_block();
+void mqtt_unblock();
+
 #endif /* MODULES_MQTT_CMD_H_ */