diff --git a/libraries/mbed/api/I2C.h b/libraries/mbed/api/I2C.h
index 9f4866e3e1..3097fb943e 100644
--- a/libraries/mbed/api/I2C.h
+++ b/libraries/mbed/api/I2C.h
@@ -141,9 +141,9 @@ public:
      *
      * @param address   8/10 bit I2c slave address
      * @param tx_buffer The TX buffer with data to be transfered
-     * @param tx_length The length of TX buffer
+     * @param tx_length The length of TX buffer in bytes
      * @param rx_buffer The RX buffer which is used for received data
-     * @param rx_length The length of RX buffer
+     * @param rx_length The length of RX buffer in bytes
      * @param event     The logical OR of events to modify
      * @param callback  The event callback function
      * @param repeated Repeated start, true - do not send stop at end
diff --git a/libraries/mbed/api/SerialBase.h b/libraries/mbed/api/SerialBase.h
index eecf98bc69..51aeb33e36 100644
--- a/libraries/mbed/api/SerialBase.h
+++ b/libraries/mbed/api/SerialBase.h
@@ -131,7 +131,7 @@ public:
     /** Begin asynchronous write using 8bit buffer. The completition invokes registered TX event callback
      *
      *  @param buffer   The buffer where received data will be stored
-     *  @param length   The buffer length
+     *  @param length   The buffer length in bytes
      *  @param callback The event callback function
      *  @param event    The logical OR of TX events
      */
@@ -140,7 +140,7 @@ public:
     /** Begin asynchronous write using 16bit buffer. The completition invokes registered TX event callback
      *
      *  @param buffer   The buffer where received data will be stored
-     *  @param length   The buffer length
+     *  @param length   The buffer length in bytes
      *  @param callback The event callback function
      *  @param event    The logical OR of TX events
      */
@@ -153,7 +153,7 @@ public:
     /** Begin asynchronous reading using 8bit buffer. The completition invokes registred RX event callback.
      *
      *  @param buffer     The buffer where received data will be stored
-     *  @param length     The buffer length
+     *  @param length     The buffer length in bytes
      *  @param callback   The event callback function
      *  @param event      The logical OR of RX events
      *  @param char_match The matching character
@@ -163,7 +163,7 @@ public:
     /** Begin asynchronous reading using 16bit buffer. The completition invokes registred RX event callback.
      *
      *  @param buffer     The buffer where received data will be stored
-     *  @param length     The buffer length
+     *  @param length     The buffer length in bytes
      *  @param callback   The event callback function
      *  @param event      The logical OR of RX events
      *  @param char_match The matching character