The nature of the synergy between Cu and ZnO has been examined using temperature-programmed desorption and reaction techniques. Adsorption of CO–CO₂–H₂ on Cu/SiO₂ and on ZnO/SiO₂ results in the formation of surface intermediates which are distinguishable by different activation energies for their dissociation into CO₂ and H₂. A mixture of the two catalysts does not simply exhibit the characteristics of both components. After adsorption of the gas-feed on a physical mixture at 325 K, CO₂ and H₂ are desorbed separately. It is proposed that, during the dissociation of the species on the Cu surface, hydrogen can spill acoross the silica, causing the zinc oxide to become hydrided and hydroxylated. Further, if there is intimate contact between the phases, it could be envisaged that the hydroxyl species contribute to the creation of bidentate Cu-methanoates from CO₂ and even CO. This would explain the observation that when CO–H₂(at 101 325 Pa) is passed over the catalysts during programmed heating, a sample prepared by coprecipitation is the only one to generate methanol.