Nonstoichiometric nickel-manganese spinel oxides, NixMn3-x square O-3 delta /4(4+delta) (1 greater than or equal tox greater than or equal to0), have been synthesized by calcination in air of mixed oxalates at 623 K. These materials are shown to be highly reactive for CO oxidation, some conversion being observed at room temperature for the most active solid (x = 1.0). The interaction of CO and O-2 With these oxides has been studied by in situ LR spectroscopy under steady-state and transient reaction conditions. A detailed mechanism is proposed wherein CO reacts with coordinatively unsaturated cations to give carbonyl complexes which in turn react with surface oxygen activated on anionic vacancies. Adsorbed and gaseous CO2 also undergo much slower side reactions with lattice oxygen or surface hydroxide groups to give more stable hydrogen carbonate and carbonates species, which lead to catalyst deactivation. Marked effects of pretreatment are explained on the basis of the observed kinetics and the proposed mechanism.