The interference transmission spectra T(lambda) at normal incidence for different thicknesses of amorphous arsenic sulfide semiconducting films deposited by thermal evaporation method were obtained in the spectral region from 400 nm up to 2500 nm. The direct analysis proposed by Swanepoel, which is based on the use of the extremes of the interference fringes in order to derive the real and imaginary parts of the complex index of refraction, and also the film thickness. The dispersion of it is discussed in terms of the Wemple-DiDomenico's single-oscillator model. In addition, the optical band gap E-g(opt) has been determined from the absorption coefficient values using Tauc's procedure, i.e. from the relationship alpha hv = K(hv - E-g(opt))(2), where K is a constant. The optical band gap is interpreted in terms of the bond-strengths of the chemical bonds present in the glass compositions under study. The results were achieved for stoichiometric composition As40S60 and non-stoichiometric composition As35S65. (c) 2006 Elsevier B.V. All rights reserved.