Exposure with bandgap light, in air, and thermal annealing at a temperature near the glass transition temperature, of thermally-evaporated amorphous As40S20Se40 thin films, were found to be accompanied by structural effects, which, in turn, lead to changes in the refractive index and shifts in the optical absorption edge. Also, clear indications of photo-oxidation were found after light exposure, in air. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple-DiDomenico model. The strong-absorption region of the absorption edge is described using the 'non-direct electronic transition' model, proposed by Tauc. Regarding the structural transformations that take place in AS(40)S(20)Se(40) chalcogenide thin films, when exposed or annealed, changes in the first sharp diffraction peak, present in the X-ray diffraction pattern, with both treatments, has been interpreted as a diminution of the interstitial volume around the AsS3-nSen (n = 0,1,2,3) pyramidal structural units, which form the amorphous network of the samples under study. This result is certainly consistent with the decrease of the average thickness found for the illuminated and annealed chalcogenide samples. (C) 2009 Elsevier B.V. All rights reserved.