The kinetics of the photo-induced solid state chemical reaction of silver with a-As33S67 films in a 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 show that there are two stages in the photo-induced reaction process. Rate coefficients and activation energies have been measured and a comparison of the composition and structure of the 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 for the Ag concentration profile during the course of the reaction and indicate a homogeneous distribution of Ag in the final reaction products, the Ag concentration being 31 at.%. The step-like form of the Ag concentration profile is discussed in the framework of the G-CAg diagram for the Ag-As33S67 system. On the basis of the back scattering results a model is suggested which explains the kinetics of the photo-induced reaction.