Detailed characteristics of photoinduced surface deposition (PSD) of metallic Ag have been studied using Ag‐As‐S bulk glasses to reveal the mechanism. The composition dependence of PSD in the AgxAs60−xS40(25≤x≤45) system shows that the photosensitivity decreases with a decrease in the Ag content and becomes negligible atx&bartil;25. All the related crystalline materials hardly exhibit the PSD, even if the Ag content is greater than 45 at. %. The size of photodeposited Ag particles and the number per unit area depend on the Ag content and illumination conditions (light intensity, photon energy, and temperature). The PSD phenomenon is induced by light with photon energy higher than the optical band gap, and the photosensitivity shows a maximum at room temperature. PSD can be suppressed by coating over the surface with thin Au film. From these results and other observations it can be suggested that the PSD phenomenon is characteristic of thermodynamically unstable glasses containing excess Ag+ions and is a kind of photoelectro‐ionic chemical reaction. ©1995 American Institute of Physics.