diff --git a/components/modules/i2c_hw_master.c b/components/modules/i2c_hw_master.c
index 74dc3312..8c81c365 100644
--- a/components/modules/i2c_hw_master.c
+++ b/components/modules/i2c_hw_master.c
@@ -102,13 +102,13 @@ static void i2c_transfer_task( task_param_t param, task_prio_t prio )
 
   if (job->cb_ref != LUA_NOREF) {
     lua_rawgeti( L, LUA_REGISTRYINDEX, job->cb_ref );
-    lua_pushinteger( L, job->err );
     if (job->result) {
       // all fine, deliver read data
       lua_pushlstring( L, (char *)job->result->data, job->result->len );
     } else {
       lua_pushnil( L );
     }
+    lua_pushboolean( L, job->err == ESP_OK );
     lua_call(L, 2, 0);
   }
 
@@ -347,21 +347,32 @@ int li2c_hw_master_transfer( lua_State *L )
     esp_err_t err = i2c_master_cmd_begin( job->port, job->cmd,
                                           job->to_ms > 0 ? job->to_ms / portTICK_RATE_MS : portMAX_DELAY );
 
-    if (err == ESP_OK) {
+    switch (err) {
+    case ESP_OK:
       if (job->result) {
         // all fine, deliver read data
         lua_pushlstring( L, (char *)job->result->data, job->result->len );
       } else {
         lua_pushnil( L );
       }
+      lua_pushboolean( L, 1 );
 
       // prepare the next transfer
       i2c_setup_ud_transfer( L, ud );
-      return 1;
-    }
+      return 2;
 
-    i2c_setup_ud_transfer( L, ud );
-    return i2c_lua_checkerr( L, err );
+    case ESP_FAIL:
+      lua_pushnil( L );
+      lua_pushboolean( L, 0 );
+
+      // prepare the next transfer
+      i2c_setup_ud_transfer( L, ud );
+      return 2;
+
+    default:
+      i2c_setup_ud_transfer( L, ud );
+      return i2c_lua_checkerr( L, err );
+    }
   }
 }
 
diff --git a/docs/en/modules/i2c.md b/docs/en/modules/i2c.md
index a29f999d..2cb4243d 100644
--- a/docs/en/modules/i2c.md
+++ b/docs/en/modules/i2c.md
@@ -129,18 +129,20 @@ Send (`SW`) or queue (`HWx`) an I²C stop condition.
 ## i2c.transfer()
 Starts a transfer for the specified hardware module. Providing a callback function allows the transfer to be started asynchronously in the background and `i2c.transfer()` finishes immediately. Without a callback function, the transfer is executed synchronously and `i2c.transfer()` comes back when the transfer completed. Data from a read operation is returned from `i2c.transfer()` in this case.
 
-First argument to the callback is the error code (0 = no error), followed by a string with data obtained from a read operation during the transfer or `nil`.
+First argument to the callback is a string with data obtained from a read operation during the transfer or `nil`, followed by the ack flag (true = ACK received).
 
 #### Syntax
 `i2c.transfer(id[, cb_fn][, to_ms])`
 
 #### Parameters
 - `id` interface id, `i2c.SW` not allowed
-- `cb_fn(err, data)` function to be called when transfer finished
+- `cb_fn(data, ack)` function to be called when transfer finished
 - `to_ms` timeout for the transfer in ms, defaults to 0=infinite
 
 #### Returns
-- `string` of received data (or `nil` if no read) for synchronous operation
+- synchronous operation:
+    - `data` string of received data (`nil` if no read or NACK)
+    - `ack` true if ACK received, false for NACK
 - `nil` for asynchronous operation
 
 ## i2c.write()