CuInSe2 and CuInS2 form a continuous solid solution in which the optical bandgap varies from 1.0 to 1.55 eV (B. Abid ct al., Proc. IEEE PVSC, 1987 1305-1308). By reacting CuInSe2 films in a flowing H2S/Ar atmosphere, the films can be completely converted to CuInS2 or converted to a graded CuIn(Se1-ySy)(2) film, depending on processing conditions. A phenomenological model of the reaction/diffusion process is presented wherein H2S reacts with CuInSe2 at the surface to form CuInS2, releasing Se. The CuInS2 and CuInSe2 layers interdiffuse, resulting in a S/Se gradient in the structure. X-Ray diffraction (112) line profiles of graded films are compositionally broadened due to continuously changing lattice parameters. The three-dimensional sulfur distribution was calculated for measured grain size distribution using the formalism of Gilmer and Farrell and used to generate diffraction line profiles. These were fit to measured line profiles using bulk and grain boundary diffusion coefficients as fitting parameters, yielding diffusion coefficients and activation energies for bulk and grain boundary diffusion processes.