A new method to determine the optical parameters of ZnxCd1-xS polycrystalline thin films is proposed. Sub-gap absorption, caused by acceptor states, is taken into account in the determination of the refractive index and film thickness. This procedure avoids errors in the results, frequently observed when the absorption is ignored. In the low and medium absorption region the refractive index is parametrized by the Wemple and DiDomenico (W-D) single oscillator model. The W-D parameters are further used in order to extrapolate the refractive index at the absorption edge to compute the absorption coefficient and the optical gap. The transmission spectra of typical CdS, (Zn,Cd)S and ZnS thin films and their corresponding values of n, alpha and E(g) are presented.