Ce–Sn–O mixed oxide films prepared by simultaneous Sn metal and cerium oxide magnetron sputtering were studied by high resolution photoemission. The analysis showed that the degree of reduction of the cerium oxide depends on the tin concentration in the film. Ce4+→Ce3+ conversion is explained by a charge transfer from Sn atoms to unoccupied orbital Ce 4f0 of cerium oxide by forming Ce 4f1 state. The X-ray Photoelectron Spectroscopy data were compared with study of the single-crystalline CeO2 thin films and Sn/CeO2(111) model system prepared and studied in situ excluding air exposure effects.