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Thin films of high-quality ZnS were grown on (001) GaAs and (111) Si substrates by metal-organic chemical-vapor deposition. 2-K reflectivity was used to analyze various samples grown at different temperatures. The spectra show two structures at 3.801 eV corresponding to the free exciton and at 3.871 eV corresponding to the E0+Δ0 transition. Theoretical reflectivity spectra were calculated using the spatial dispersion model with two oscillators. Thus, the transverse energies, the longitudinal transversal splitting, the oscillator strengths, and the damping parameters were determined for both the free exciton and the split-off exciton of ZnS. Photoluminescence measurements were also carried out using an excimer laser (308 nm). Both light-hole and heavy-hole excitons were observed, which allow for the determination of the strain that exists in the layer. This strain is due only to the difference between the thermal-expansion coefficients of the GaAs substrate and the ZnS epilayer, and was demonstrated to be a tensile strain. This paper also presents results on the band-gap energy variation as a function of the temperature and photoluminescence spectra when the excitation was varied from weak to very high densities (15 MW/cm2).