The deactivation of Cu/ZnO catalyst during methanol dehydrogenation to form methyl formate has been studied. The Cu/ZnO catalyst was seriously deactivated under the reaction conditions : various temperatures of 493, 523 and 553 K, atmospheric pressure and methanol GHSV of 3000 mi (STP)/g-cat h. The weight loss due to reduction of ZnO in the Cu/ZnO catalyst was monitored by a microbalance. X-ray induced Auger spectroscopy of Zn(L(3)M(4,5)M(4,5)) showed the increase in the concentration of metallic Zn on the catalyst surface after the reaction. Temperature-programmed reduction (TPR) of the Cu/ZnO catalyst with methanol demonstrated that the reduction of ZnO in Cu/ZnO was suppressed by introduction of CO2 into the stream of helium-methanol. As the concentration of CO2 in the feed gas increased, the weight loss of the Cu/ZnO catalyst due to the reduction of ZnO decreased. The deactivation of the Cu/ZnO catalyst in the methanol dehydrogenation was also retarded by the addition of CO2. In particular, oxygen injection into the reactant feed regenerated the Cu/ZnO catalyst deactivated during the reaction. Based on these observations, the cause of deactivation of the Cu/ZnO catalyst has been discussed.