The effects of the pretreatment temperature of Co₃O₄/γ-Al₂O₃ catalyst and the presence of a trace amount of water in the reactant stream on the reactivity in the low temperature CO oxidation have been investigated. The presence of ca. 5 ppm of water in the reactant stream was found to result in a notable deactivation of reactivity of the Co₃O₄/γ-Al₂O₃ catalyst, when the catalyst was pretreated at temperatures below 500 ℃. In contrast, the presence of this small amount of water had no influence on the catalyst activity when the pretreatment temperature is elevated to 550 ℃. In the reaction temperature range of .15-40 ℃, a decrease of catalytic activity with increasing reaction temperature is observed. This means the existence of negative activation energy.