Films of amorphous Se, Ge15Se85, and Ge25Se75 were prepared by thermal evaporation. Measurements of photodarkening at 78 K showed that the optical gap (E-g) and the slope of optical absorption edge (B) are intercorrelated. The correlation between the photoinduced changes of E-g and B is discussed within both the: model of Davis and Mott and Tauc's model for optical transitions in amorphous solids. (C) 1998 Elsevier Science B.V. All rights reserved.