The results of an investigation of the evaluation of energy signal degradation of anM-ary p.s.k. system due to intersymbol interference and fading are analysed.Attention is concentrated upon the influence of intersymbol interference caused by the limited band of the receiver filter as well as the transmitter one. As far as the channel is concerned, it has been assumed there is nonselective fading in it.Part 1 presents the analysis method being applied;Part 2 analyses the energy degradation characteristics ofM-ary signals resulting from intersymbol interference in the case when no fading occurs in the channel;Part 3 analyses the influence of intersymbol interference and fading jointly.