The brittle-to-ductile transition observed in As-S(Se) films under the action of light has been investigated. In this the bandgap light incident on the chalcogenide film causes a reversible transition from brittle to ductile state (the photoplastic effect) produced by successive on-off cycles of band-gap illumination [M.L. Trunov, A.G. Anchugin, Sov. Tech. Phys. Lett. 18 (1992) 37, and references therein; M.L. Trunov, A.G. Anchugin, Sov. Tech. Phys. Lett. 18 (1992) 78; M.L. Trunov, A.G. Anchugin, N.D. Savchenko, V.M. Rubish, Proc. SPIE 2113 (1994) 183]. A macroscopic model is proposed to explain this effect in terms of photosoftening phenomena of glasses structure, which causes an essential decrease in glasses viscosity to the level close to 10(12)-10(13) P. This effect is electronic, not thermal in origin and consists of two parts, namely negative (a sharp decrease in viscosity to 1 x 10(12) P) and positive (its increase to 10(13)-10(14) p) stages. These parts are temporary and exist under irradiation only. We assume that the first part is general and causes all photostructural and related phenomena in chalcogenide glasses. (C) 2003 Elsevier B.V. All rights reserved.