Amorphous Ga20S75Sb5 and Ga20S40Sb40 thin films were prepared onto glass substrates by using thermal evaporation method. The effect of annealing (under vacuum) at different temperatures on the optical parameters was investigated in the temperature range 373-593 K. The optical absorption coefficient (alpha) for the as-deposited and annealed films were calculated from the reflectance and transmittance measurements in the range 190-900 nm. X-Ray diffraction indicates that the as-deposited films and those annealed up to the glass transition temperature (T-g) exhibit amorphous state. On annealing above the glass transition temperature these films show a polycrystalline structure. Analysis of the optical absorption data indicates that the optical band gap E-g(opt) of these films obeys Tauc's relation for the allowed non-direct transition. It was found that the optical band gap E-g(opt) increases with annealing temperature up to T-g, whereas above T-g there is a remarkable decrease. The obtained results were interpreted on the basis of amorphous-crystalline transformation. (C) 2003 Elsevier B.V. All rights reserved.