The EPR spectra of freshly prepared CeO2 samples as well as of samples used as a catalyst in the oxidation of CO to CO2 are reported. Attribution of these patterns to oxygen species O-2(-), O-2, O- or to Ce3+ ions is discussed and dynamic phenomena involving these species are described. Bottlenecked systems between conduction electrons and Ce3+ ions are envoked to explain the observation of the Ce3+ EPR spectrum up to temperatures of about 370 K. Oxygen-deficient systems are obtained when the sample is used as a catalyst for CO oxidation in the presence of pyrex or quartz powder as a diluent, showing that the latter can play some role in stabilizing such species.