CdS layers were epitaxially grown on GaAs in an atmosphere of hydrogen gas by the close‐spaced technique. Then the thermodynamics of the reaction, the morphology, and electrical properties of the CdS epitaxial films were studied. Examinations of the temperature dependence of the evaporation rate of source CdS powder, of film growth rates, and of surface morphology demonstrated that the growth kinetics are between the mass‐transfer control case and the surface‐reaction control case, but closer to the former. The electrical properties of the films were greatly influenced by growth conditions, especially by substrate temperature. Film resistivity increased from 10−3to 1 &OHgr; cm as a function of substrate temperature, and the carrier concentration in the film increased exponentially as the temperature of the substrate was raised: Carrier concentration = 1.31×1029exp (−1.98/kTsub) cm−3. The dominant donor species in CdS films was shown to originate from the autodoping of Ga by the GaAs substrate. Electron mobility in the films also increased as the temperature of the substrate was raised, which was attributed to the superior crystallinity in the films grown at a rather high substrate temperature.