The kinetics of photo-induced solid state chemical reaction of silver with a As 33S 67 films in conventional sandwich structure was measured by monitoring the change in thickness of the undoped chalcogenide using a modified computer-controlled technique. The kinetic data obtained shows that there are two stages in the photo-induced reaction process. Rate coefficient and activation energies have been measured and a comparison of the composition and structure of reaction products detected by Rutherford backscattering spectroscopy (RBS) has been made. The RBS spectra obtained at different stages of the photo-induced reaction process show a step-like form of the Ag concentration profile during the course of the reaction and the homogeneous distribution of Ag in the final reaction products, e.g. 31 at percent Ag. The step-like form of the Ag concentration profile is discussed in the framework of the G-c Ag diagram for the Ag-As 33S 67 system. On the basis of the RBS results, it is suggested model explaining the kinetics of the photo-induced reaction.