In situ infrared studies of methanol synthesis from H-2/CO2 have been conducted over Cu/SiO2 promoted with ZrO2, TiO2, and a mixture of TiO2/ZrO2, with the aim of establishing the means by which the added oxide influences the activity of the catalyst for methanol synthesis. While ZrO2 and TiO2 individually enhance the methanol synthesis activity of Cu/SiO2, a significantly greater enhancement is achieved by addition of both oxides simultaneously. In the absence of oxide addition, infrared spectra taken under reaction conditions suggest that methanol synthesis from CO2 takes place on the surface of Cu exclusively. When ZrO2 is added to the catalyst, there is strong evidence for methanol synthesis on the surface of both Cu and ZrO2. Catalysis on the surface of ZrO2 occurs as a consequence of CO2 adsorption to form b-HCO3-Zr, which then undergoes progressive hydrogenation to form methanol. The required atomic hydrogen is supplied from the surface of Cu particles by spillover. When TiO2 is added there is no evidence for catalysis occurring on the oxide surface, but strong evidence for an enhancement in the level of reaction occurring on the surface of Cu. The observed increase in the rate of methanol synthesis is attributed to a reconstruction of the Cu particles to expose the (100) surface, which have been demonstrated to be significantly more active than the (110) or (111) surfaces. The infrared spectra observed when a mixture of ZrO2 and TiO2 is added to Cu/SiO2 resembles a composite of the spectra observed upon addition of the individual oxides, and hence, the enhanced rate of methanol synthesis is ascribed to the combined effects of ZrO2 and TiO2.