Thin films of Cd0.9Zn0.1S and CdS were prepared by thermal evaporation under vacuum of 10(-6) Torr and with deposition rate of 60 nm/min. X ray diffraction studies confirm the hexagonal structure of both CdS and Cd0.9Zn0.1S films. The effect of heat treatments with or without CdCl2 enhances the grain size growth and improves the crystalline of the films. Moreover, the activation energy is decreased by heat treatment with or without CdCl2 for all thin films. The optical absorption coefficient of Cd0.9Zn0.1S thin films were determined from measured transmittance and reflectance in the wavelength range of 300 to 2500 nm. The optical absorption spectra reveal the existence of direct energy gap for these films. It was found that the optical energy gap decreases upon annealing or CdCl2 treatments. (C) 2003 Kluwer Academic Publishers.