Zn(S,O) thin films was elaborated by annealing in air and sulfur atmospheres the zinc films deposited by thermal evaporation method. The samples were analyzed for their optical properties by using Ultraviolet-Visible-near Infrared spectroscopy. The optical constants and the dispersion parameters of the films were calculated from the analysis of the transmittance and reflectance data in the spectral range 300 - 1800 nm. The band gap energy varied from 3.27 to 3.08 eV depending on the sulfur content. The bowing parameter was calculated as 2.45 eV. Swanepoel model was employed to study the wavelength dependence of the refractive index. The lattice dielectric constant epsilon(L), the plasma frequency cop and the ratio of the carrier density to the carrier effective mass N/m* were calculated from the examination of the refractive index. Wemple-Di Domenico single oscillator model was applied to determine the dispersion parameters of the samples such as the high-frequency dielectric constant e(infinity), the oscillator energy E-0 and the dispersion energy E-d. (C) 2018 Elsevier B.V. All rights reserved.