In f.m. reception, the delayed signals caused by multi-path propagation result in unwanted amplitude and phase modulation of the primary signal, and consequent distortion of the programme output of the receiver. The paper describes the results of tests which were carried out in simulated multi-path conditions to determine the importance of the parameters of both the received signal and the receiver in influencing the subjective annoyance caused by the distortion.With a well-designed receiver, the distortion of piano music is ‘slightly disturbing’ when a single delayed signal is present having an equivalent path difference of 8 km and an amplitude of 35% relative to the primary signal. For a path difference of 29 km, however, the permissible relative amplitude is only 6% for the same subjective annoyance. Under the same conditions the distortion of speech is imperceptible. In comparison, receivers providing inadequate suppression of the unwanted amplitude modulation are much more susceptible to the distortion.The use of pre-and de-emphasis appreciably reduces the distortion, being equivalent to a reduction of about 8 dB in the amplitude of the delayed signal when the path difference is about 16 km. Similarly, the distortion is less noticeable when the loudspeaker has a poor response at high audio frequencies.The mechanism of multi-path distortion is discussed, and the harmonic spectra of the distortion shown for particular conditions.