Cerium oxide cluster ions, CenO2n+x+ (n = 2-9, x = -1 to +2), were prepared in the gas phase by laser ablation of a cerium oxide rod in the presence of oxygen diluted in He as the carrier gas. The stable stoichiometry of the cluster ions was investigated using a mass spectrometer in combination with a newly developed post heating device. The oxygen-rich clusters, CenO2n+x+ (x = 1, 2), were found to release oxygen molecules, and CenO2n+x+ (x = -1, 0) were exclusively formed by post heating treatment at 573 K. The CenO2n-1+ and CenO2n+ clusters were found to be thermally stable, and the oxygen-rich clusters consisted of robust CenO2n-1+ and CenO2n+ and weakly bound oxygen atoms. Evaluation of the reactivity of CenO2n+x+ with CO molecules demonstrated that CenO2n+ oxidized CO to form CenO2n-1+ and CO2, and the rate constants of the reaction were in the range of 10(-12)-10(-16) cm(3) s(-1). The CO oxidation reaction was distinct for n = 5, which occurred in parallel with the CO attachment reaction.