Substitutional solid solutions in the CdS-PbS system were obtained by aging of chemically deposited cadmium sulfide thin film in an aqueous solution of lead acetate Pb(C2H3O2)(2). By using the thermodynamic calculations a possibility of proceeding of a heterogeneous ion-exchange reaction at the interface "chemically deposited CdS film/lead salt aqueous solution" was estimated. The effect of solid-state diffusion on the morphology, composition, and optical properties of the CdS films was investigated. By means of X-ray photoelectron spectroscopy with layer-by-layer sputtering, the chemical composition across the obtained films thickness was analyzed, and concentration depth profiles of the elements were plotted. The coefficient of the lead diffusion into the cadmium sulfide film during its ion-exchange transformation was calculated.