Demodulation of M-ary quadrature amplitude modulation (M-QAM) requires the receiver to estimate the phase and amplitude of the received signal. The demodulator performance is sensitive to errors in these estimates, and the sensitivity increases as M increases. We examine the effects of phase and amplitude errors on the performance of QAM communication systems with error-control coding and soft-decision decoding. A mathematical analysis of these effects is presented for two soft-decision decoding metrics. Performance comparisons are given for 16-QAM and 64-QAM for two error-control coding techniques and two soft-decision decoding metrics.