Cu-Zn-Al (45/50/5) catalyst (Cat) and Cu-Zn-Al-Li (40/50/5/5)catalyst (Cat-Li) have been prepared and employed for methanol synthesisfrom CO/H2 and CO2/H2, respectively. Techniques/methods such as TPD,TPR, TPSR, pulsing, CD3I-trapping, isotope labeling, EPR, and in situ DRIFTwere adopted to characterize the catalysts and to study the reactionmechanism. Catalysts in the states of H-eliminated, H-containing, and O-containing were investigated comparatively to clarify the effects of surface Oand H on the activation of CO2 and CO. A CO/CO2 hydrogenationmechanism evidenced by the results of tormate and formaldehydehydrogenation as well as methanol oxidation has been proposed. Due to thegeneration of oxygen vacancies by the substitution of Li+ for Cu2+ in the CuOlattice, the addition of Li+ to Cat improved the methanol synthesis activity.CO2 could be activated by a trapped electron (F-center) and the CO2-generated could be hydrogenated readily to formate and methylenebisoxy; thedecomposition of methylenebisoxy would give rise to H2CO and surfaceoxygen. CO could be activated by surface oxygen and the CO2' producedfollowed the route of CO2 hydrogenation. In CD3I-trapping experiments, wecaptured surface oxygen. In the absence of surface oxygen, CO might behydrogenated directly to formyl, a way inferior to that of CO2 hydrogenation.The surface mode of H2CO resulted from methylenebisoxy decompositionmay be different from that resulted from formaldehyde adsorption or COhydrogenation.