Abstract-We investigate the error performance of a communication system where a nonuniform memoryless binary source is transmitted via Gray-mapped -ary phase-shift keying or quadrature amplitude modulation over memoryless Rayleigh fading channels, and demodulated via optimal maximum a posteriori detection. Using recently derived upper and lower bounds on the probability of a general union of events, which are tight and can be efficiently computed, the system symbol-error ( ) and bit-error ( ) rates are evaluated for a wide range of channel conditions. Since for nonuniform signaling, Gray mapping is not necessarily optimal for minimizing or (as was recently shown by Takahara et al.), we also evaluate the system performance under the map obtained by Takahara et al. and compare it with a Gray-mapped system. Index Terms-Bit and symbol error rates, Gray mappings, lower and upper bounds, maximum a posteriori (MAP) decoding, nonuniform sources, phase-shift keying (PSK) and quadrature amplitude modulation (QAM) modulations, probability of a union, Rayleigh fading channels.