Multiconfigurational quantum chemical methods (CASSCF/CASPT2) have been used to study the chemiionization reactions Ce + O → CeO+ + e- and Ce + O2 → CeO2+ + e-. Selected spectroscopic constants for CeOn and CeOn+ (n = 1,2), as well as reaction enthalpies of the chemiionization reactions of interest, have been computed and compared with experimental values. In contrast to the lanthanum case, for both Ce + O2(X3 Σg-) and Ce + O2(a1 Δg), the Ce + O2 → CeO2+ + e- reaction is shown to be exothermic, and thus, contributes to the experimental chemielectron spectra. The apparent discrepancy between the computed reaction enthalpies and the high kinetic energy offset values measured in the chemielectron spectra is rationalized by arguing that chemielectrons are produced mainly via two sequential reactions (Ce + O2 → CeO + O, followed by Ce + O → CeO+ + e-) as in the case of lanthanum. For Ce + O2(a1Δg), a chemielectron band with higher kinetic energy than that recorded for Ce + O2(X3Σg-) is obtained. This is attributed to production of O(1D) from the reaction Ce + O2(a1Δg) → CeO + O(1D), followed by chemiionization via the reaction Ce + O(1D) → CeO+ + e-. Accurate potential energy curves for the ground and a number of excited states of CeO and CeO+ have been computed, and a mechanism for the chemiionization reactions investigated experimentally was proposed.