- Abstract Structural and chemical changes in C 60 thin films, grown on a Cu substrate and exposed to air or I 2 atmosphere, at room temperature, were monitored by X-ray diffraction, Auger electron and X-ray photoelectron spectroscopy. Exposure to air is demonstrated to result not only in oxygen diffusion but also in a counter diffusion of the metal from the substrate into the C 60 matrix. In particular, 10 months of air-exposure led to the presence of Cu and O atoms at all depths over the sample and the formation of a complex which we refer to as Cu x O y C 60 . The new phase is quasi-stable at room temperature, but a relatively short thermal annealing at 150°C destroys it and restores the initial C 60 FCC lattice. After 10 min of exposure of the Cu-supported C 60 films to an I 2 atmosphere the Cu sub-layer disappears completely and macroscopic amounts of a stable CuI phase are formed over the entire thickness of the C 60 film. To explain the results a model of chemically-induced counter electro-diffusion is proposed. We propose the possible usefulness of this counter diffusion approach for tailoring C 60 -based materials doped with various compounds in the form of both phase-separated composites and solid solutions (intercalated fullerides).