Chalcogenide glasses exhibit many attractive optical properties, including high transmission in IR region, a high refractive index and reversible photodarkening. Irradiation of amorphous chalcogenide films with near band-gap light causes a red shift of the optical absorption edge and a corresponding increasing of the refractive index [1]. The initial state can be recovered by annealing near the glass transition temperature, i.e. the process is reversible. In the present paper the influence of the composition and film thicknesses, on the photodarkening process in amorphous As100-xSex (x=40÷98) is presented. The relaxation of the relative optical transmission T(t)/T(0) is shown in Fig. 1. The observed relaxation of photodarckening may be described by the stretched exponential function T(t)/T(0) = A0+Aexp[-(t-t0)/τ](1-α), where t is the exposure time, τ is the apparent time constant, A characterizes the exponent amplitude, t0 and A0 are the initial coordinates, and α is the dispersion parameter (0<α<1). The above parameters were calculated for all investigated amorphous films using a computer fitting. It was established, that the higher sensitivity to light exposure exhibit the non-stoichiometric As50Se50, As55Se45, and As60Se40 amorphous films, and decrease with increasing of Se content in the As100-xSex glass system. The experimental results are interpreted in terms of structural optical polymerization process.figura