Amorphous Se82-xTe18Sbx thin films with different compositions (x = 0, 3, 6 and 9 at.%) were deposited onto glass substrates by thermal evaporation. The transmission spectra, T(lambda), of the films at normal incidence were obtained in the spectral region from 400 to 2500 nm. Based on the use of the maxima and minima of the interference fringes, a straightforward analysis proposed by Swanepoel has been applied to derive the optical constants and the film thickness. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple and DiDomenico model. Tauc relation for the allowed non-direct transition describes the optical transition in the studied films. With increasing antimony content the refractive index increases while the optical band gap decreases. The optical band gap decreases from 1.62 to 1.26 eV with increasing antimony content from 0 to 9 at.% The chemical-bond approach has been applied successfully to interpret the decrease of the optical gap with increasing antimony content. (C) 2008 Elsevier B.V. All rights reserved.