Zinc oxide powders have been prepared by the isovalent substitution method by means of high-temperature annealing of a zinc sulfide powder synthesized by a chemical method. Investigation of photoluminescence (PL) spectra in dependence on annealing temperature varied from 870 to 1050С has made it possible to find the temperature range of 920–950С for the most active isovalent substitution of oxygen atoms for sulfur atoms and the formation of a ZnOS phase. X-ray diffraction analysis has revealed that the powder with crystal lattice parameters ао = 0.3249 nm and со = 0.5206 nm and the ratio of со/ао = 1.60, which are characteristic of zinc oxide crystals, is synthesized at the maximum annealing temperature. It has been found that the maximum of a structureless PL band in the spectrum for the ZnO powder at room temperature is localized at 500 nm. It has been supposed that the band is caused by ―free electron–acceptor‖ radiative transitions, where the acceptor level is 0.95 eV above the valence band edge.