Abstract: A method and an apparatus for detecting and identifying a contaminant, such as ice, on the surface of a structure, such as a wing, are disclosed. The apparatus has a probe having a delay block with an interface apt to be placed flush with the surface of the structure and two faces at the same angle of inclination relative to the interface and having transmission and reception transducers each oriented perpendicularly to one of the faces. The block has an acoustic impedance lying in the range of 0.7 to 3.times.10.sup.6 kg/m.sup.2.s. It further has an energization and analysis unit for applying an electrical energization pulse to the transmission transducer and comparing the amplitude and the phase of the echo from the interface with a reference echo corresponding to no contamination.