Photoinduced changes in refractive index and thickness in evaporated As2S3films have been measured by means of a prism‐coupling technique. The irreversible and reversible structural transformations are associated with increases in the refractive index of 0.133 and 0.029, respectively, at a wavelength of 633 nm. Additionally, a dynamical change in refractive index by 0.003 appears under the band‐gap illumination of about 10 mW/cm2. The origin of this change is attributable to a result from photoelectronic excitation. In the irreversible process, the films expand first and subsequently contract as the refractive index increases monotonically, whereas any change in thickness was not detected within an experimental error of ±10 A˚ for the reversible process.