We have studied the stoichiometry of cerium oxide films deposited by RF sputtering as a function of deposition conditions using the resonant Rutherford backscattering method. We found that some films have an off-stoichiometry of CeOy with y greater than 2.0. Such an off-stoichiometry cannot be due to a mixture of the known phases of bulk cerium oxide samples. Those thin samples may have either cerium vacancies or interstitial oxygen atomic impurities. We have tried to determine the valence of the cerium ions by measuring the X-ray photoemission.