Growth characteristics of CdS layers on (1 0 0) GaAs have been studied in low-pressure metalorganic vapor-phase epitaxy. CdS layers grow in cubic modification dominantly in the temperature range from 250 degreesC to 350 degreesC. At the growth temperature above 350 degreesC, hexagonal modification becomes dominant with increase of the growth temperature. High-quality single-crystal cubic CdS layers were obtained by employing the two-step growth; first, single-crystal cubic CdS layers were grown at 300 degreesC as buffer layers, and then the second layers were grown at 450 degreesC. The grown layers were evaluated by photoluminescence and Hall measurements. The layers showed sharp (A(0), X) and (D-0, X) lines at 2.534 and 2.546 eV. Electron density of these layers was 2.0 x 10(15) cm(-3) at 300 K. Low resistive n-type layers were obtained by iodine doping. The electron density of doped layers was controlled from 5.0 x 10(16) to 6.5 x 10(19) cm(-3) with the ethyliodine supply rate. The ionization energy of iodine donor was estimated to be 47 meV in cubic CdS layers.