Abstract: Demodulation of M-ary continuous-phase FSK signals is effected in accordance with phase measurements taken at symbol transition times. The phase error caused by intersymbol interference is estimated by knowledge of the type and degree of distortion which is principally dependent upon the frequency difference between successive data symbols and phase distortion characteristics of the channel filter. Pairs of phase nodes are selected at each symbol transition time and, from each pair, a tentative choice is made by using the expected phase distortion based on adjacent phase node pairs. Refined choices are then made using the estimated phase distortion based on adjacent tentative choices. Differences between successive tentative choices of phase node selections yield a sequence of frequencies making up the FSK signal. Circuitry implementation embraces the use of phase-locked loop detectors and digital decision logic including programmable read-only memories.