The transmission spectra of bulk and thin films of (As 2 S 1.5 Se 1.5) 1-x:Sn x in the visible and near infrared (IR) regions were investigated. Doping of As 2 S 1.5 Se 1.5 chalcogenide glass with tin impurities essentially reduce the absorption bands of SH (Se-H) and H 2 O located at ν = 5190 cm -1 and ν = 3617 cm -1, respectively. The amorphous As 2 Se 3:Sn x and (As 2 S 1.5 Se 1.5) 1-x:Sn x thin films exhibit photoinduced effects under the light irradiation with photon energy above the optical band gap (hν≥E g), that make its perspective materials for registration of optical and holographic information. The modification of optical parameters (optical band gap E g, absorption coefficient α, refractive index n) under light irradiation and heat treatment of the amorphous thin films with different amount of Sn was studied. The shift of the absorption edge after light exposure to lower energy region was observed, i.e. the effect of photodarkening take place. The dispersions curves n=f(λ) show a modification of the refractive index n under light exposure. For the glass composition (As 2 S 1.5 Se 1.5) 0.96:Sn 0.04 the change of the optical band gap Eg opt under light exposure was determined from 1.92±0.02 eV to 1.86±0.02 eV. The similar calculations of the optical constants were done for the amorphous films of glass compositions x=0.03 and x=0.05. The relaxation of photodarkening in amorphous As 2 Se 3:Sn x and (As 2 S 1.5 Se 1.5) 1-x:Sn x thin films, which is described by the stretch exponential function T(t)/T(0) = A 0 +Aexp[-(t-t 0)/τ] (1-α) also wasinvestigated. The experimental results are interpreted in framework of the model of molecular structure of chalcogenide glasses doped with tin impurities.