In situ Raman spectroscopy of adsorbed dioxygen was used to characterize electron defects on the surface of nanocrystalline cerium oxide that was partially reduced with H-2 and CO. Via O-16/O-18 isotope substitution, the bands in the range of 1135-1127 and 877-831 cm(-1) were assigned to the O-O stretching vibration of dioxygen species bound to one- and two-electron defects on the CeO2 surface to form superoxide (O-2(-)) and peroxide (O-2(2-)) species, respectively. A band at 357 cm(-1) was attributed to the cerium-oxygen vibration of the adsorbed superoxides, O-2(-), whereas the bands at 538 and 340 cm(-1) were assigned to the asymmetric and symmetric cerium-oxygen vibrations of the surface peroxides, O-2(2-), respectively. The dynamics of the defect annihilation that results from surface reoxidation by adsorbed dioxygen species during temperature-programmed experiments allowed peroxide species adsorbed on isolated and aggregated two-electron defects to be distinguished. A general approach to investigate the reactivity of different surface dioxygen species toward reductants was demonstrated using CO oxidation as a probe reaction.