Thin arsenic trisulfide films were deposited by thermal evaporation and thermally activated silver diffusion into these films was studied. UV-vis transmission and Rutherford backscattering spectrometry (RBS) was used for characterization of film property changes with Ag incorporation. The straightforward analysis proposed by Swanepoel, which allows determination of the dispersion relation and the film thickness from the optical transmission spectrum, was extended and applied to the chalcogenide thin films. RBS shows that silver diffusion mechanism depends strongly on an eventual annealing between the chalcogenide and silver deposition step. Without annealing the silver diffusion takes place by exchange between silver and arsenic atoms whereby sulfur atoms remain stationary, and arsenic accumulation occurs at the surface. For annealed samples silver diffuses without changing the film stoichiometry. In this case, the diffusion is much slower and the profile is more step-like. The possibility of using this technique for channel waveguide fabrication is demonstrated.