The transmission spectra were used to obtain an efficient parameterization of the spectral dependences of the optical constants of amorphous As-S thin films by applying a suitable dielectric function model. For studying the compositional dependence of the optical constants, different compositions of AsxS100-x (x=10, 15, 20, 25,30 and 40 at%) thin films were deposited by thermal evaporation technique in a base pressure of 7.5 x 10(-6) Torr at room temperature. The transmission spectra (measured in the wavelength range of 0.2-0.9 mu m) were analyzed by applying O'Leary, Johnson, and Lim (OJL) model based on the joint density of states (JDOS) functions. However, the best fit of the optical data was obtained by considering the two-layer configuration film; the top layer was assumed to be consisted of a bulk AsS material embedded in voids (air). Therefore, OJL model along with Bruggeman effective-medium approximation (BEMA) model was used to determine the effective optical constants of the As-S thin films. The photon energy dependence of the dielectric function, epsilon=epsilon(r)-i epsilon(i) of the investigated As-S films was presented. The film thickness, absorption coefficient a, refractive index n, extinction coefficient k, static refractive index n(0) and optical band gap E-g have been deduced. It was found that with the increase in arsenic content up to the stoichiometric As40S60, the indirect optical energy gap decreases, while the refractive index increases. (C) 2011 Elsevier B.V. All rights reserved.