A series of experiments under nonfading and simulated-fading conditions has produced curves showing how bit error rate (b.e.r.) varies as a function of carrier frequency offset in a quasisynchronous data transmission scheme. Both direct and subcarrier f.s.k. modulation have been used and the experiments have led to an understanding of the physical mechanisms which cause errors. Click noise is the major cause in the nonfading case and at low carrier offsets with fading. At higher offsets, the response of the discriminator to the individual fades becomes dominant, with f.m. capture effect also having an influence. The error rate depends crucially on the response of the data demodulator to these effects, and different types of demodulator can be expected to produce quite different error rates.