Thermally and optically induced irreversible changes in the optical gap and refractive index were studied for sulphur rich, nearly stoichiometric and sulphur poor Ge-As-S amorphous films prepared by thermal evaporation. For films studied the optical gap in the virgin state decreases from 2.55(9) (Ge(0.121)AS(0.172)S(0.707)) to 1.63(2) eV (Ge(0.254)AS(0.294)S(0.452)) and simultaneously the refractive index increases from 2.21 to 2.87, respectively. The most sensitive composition to illumination seems to be nearly stoichiometric film (Ge0.153As0.201S0.646), where the blue shift of the gap is observed close to 150 meV. Sulphur poor film (Ge(0.254)AS(0.294)S(0.452)) was found insensitive to illumination. Highest thermally induced blue shift of the gap, close to 250 meV, we observed just for Ge0.254As0.294S0.452 film and for this film it was observed also nearly "giant" decrease in refractive index from 2.85 to 2.42. The behaviour of Ge0.254As0.294S0.452 film is qualitatively discussed using the concept of network rigidity (insensitivity to illumination) and assuming thermally induced changes in bonding arrangement (refractive index changes). (C) 2008 Elsevier B.V. All rights reserved.