Effect of light irradiation on evaporated chalcogenide glass films of an As&sngbnd;S&sngbnd;Ge system has been studied. In order to make quantitative measurements of refractive‐index change resulting from light irradiation, light diffraction efficiency from the phase grating produced in the films with the technique of holographic storage has been measured. The maximum refractive‐index change of 0.056 has been obtained in an As2S3film. This value is two orders of magnitude higher than that of Fe‐doped LiNbO3single crystals. Light irradiation onto the films is considered to have two competing effects, one being thermal and the other purely optical.