The Ag content in Ag–As–S glasses can be spatially modified by illumination of visible light. Optical and thermal studies demonstrate that the Ag‐content modification is caused by Ag+ionic migration from the dark to the illuminated region in a sample. The migration can be reversed by changing the illuminated region. The magnitude of the Ag‐content modification is maximal (∼7 at. %) at a composition of Ag25As25S50. The migration mechanism can be accounted for by assuming diffusion of photoexcited holes, and the composition dependence can be understood on the basis of the Owens’ thermodynamic model. ©1995 American Institute of Physics.