Ultrasonic measurement apparatus 10 includes a transmitter 14 to transmit an ultrasonic signal 16 into an item 12. A receiver 18 receives echoes 20 from the item 12. The apparatus 10 measures the echo 20 and makes a comparison with another echo to extract information relating to a deposit of additional material at a surface of the item 12. For example, tribofilms or deposits may grow on the surface of the item 12 during use. A comparison of the echo, with previous echoes, allows the additional material to be detected and/or characterised.