Document inspection system comprising a transport system for transporting documents along a transport path (12) through an inspection station. At least one ultrasonic inspection apparatus at the inspection station, the apparatus including ultrasonic transmitting and receiving transducers (42,44) arranged on opposite sides of the transport path, and a processing system for monitoring ultrasonic signals received by the receiving transducer. An ultrasonic absorbing material (300) is provided around the transducers and facing the transport path for absorbing Document guide apparatus (312,314), having a lower coefficient of friction than the absorbing material, extends partially over the absorbing material (300) so as to prevent documents contacting the absorbing material in use while leaving the absorbing material exposed to the transport path at least adjacent the transducers.