The oxidation of CO over Co3O4/gamma-Al2O3 at room temperature (21 degrees C) has been studied in a continuous-flow reactor system with oxidising and reducing pretreatment, at varying stoichiometric ratios, and by using isotope-labeled O-18(2), The fresh catalyst was characterised by temperature-programmed reduction and temperature-programmed oxidation. CO deactivates the oxidised catalyst but the rate of deactivation can be suppressed by having a high O-2/CO ratio. The isotope study showed that the oxygen participating in the oxidation of CO comes from oxygen bound to the cobalt oxide surface. Finally, a mechanism for the CO oxidation over Co3O4/gamma-Al2O3 is proposed.