Glassy substrates Se85Te15 thin films were thermally evaporated onto chemically cleaned glass. Optical absorption measurements were carried out on as-deposited and thermal annealed Se85Te15 films. It was found that the mechanism of the optical absorption follows the rule of non-direct transition. The annealed Se85Te15 films showed a decrease in the optical energy gap with increasing temperature of annealing higher than the glass transition temperature (343 K). The microstructure of the as-deposited and annealed Se85Te15 films was investigated. The crystalline phases, resulting from differential thermal analysis, have been confirmed using X-ray diffraction and transmission electron microscopy. The effect of annealing on the activation energy for conduction was also studied. The results are discussed on the basis of amorphous-crystalline transformations. (C) 2003 Elsevier Science B.V. All rights reserved.