Abstract A theoretical analysis of the excitation spectra of ZnS phosphors is given. The analysis is based on the Schön—Klasens model and assumes a constant quantum efficiency, being independent of the exciting photon energy. The diffusion and surface recombination of carriers generated by photon excitation are considered. The drop of sensitivity on the high energy side of the excitation spectrum is explained by the surface recombination of carriers. The long wave tails of the excitation spectra are associated with center (activator) excitation and they are determined by optical effects. Generalized curves are derived for the evaluation of the experimental excitation spectra.