Recent cyclotron resonance expts. in n-type CdS at ultra-high magnetic fields revealed a well-pronounced max. of the electron cyclotron mass as a function of temp. These exptl. data are interpreted within a model which takes into account the interaction of the conduction electrons with LO phonons and with nonpolar optical phonons. The piezoelec. electron-phonon interaction and the electron scattering by randomly distributed defects with a short-range potential are also treated. The obsd. behavior of the cyclotron mass can be explained in terms of an interplay between several scattering mechanisms.