Abstract: Co₃O₄/CeO₂ catalyst was prepared by co-precipitation-oxidation method. Adsorption of carbon monoxide and the co-adsorption of CO/O₂ over Co₃O₄/CeO₂ sample were investigated. In situ FT-IR analysis of the mechanism in CO oxidation was interpreted over Co₃O₄/CeO₂ sample. Effects of the reaction conditions on the catalytic performance over Co₃O₄/CeO₂ catalyst for carbon monoxide oxidation was investigated at humid and dry conditions. Results indicated that the addition of cerium to the cobalt oxide was beneficial for CO oxidation over Co₃O₄ at humid condition. The formation of carbonate species with reaction on Co₃O₄/CeO₂ composite oxide slightly deactivated the catalyst. When the temperature was as low as 453 K, the dynamic equilibrium of formation and decomposition of carbonate species existed in Co₃O₄/CeO₂ composite oxide catalyst. The carbonate species disappeared above 493 K. The strong interaction between CeO₂ and Co₃O₄ may contribute to the excellent water resistance for low-temperature CO oxidation over Co₃O₄/CeO₂ catalyst at humid condition.