The effect of zirconia promotion on Cu/SiO2 for the hydrogenation of CO and CO2 at 0.65 MPa has been investigated at temperatures between 473 and 573 K. With increasing zirconia loading, the rate of methanol synthesis is greatly enhanced for both CO and CO2 hydrogenation, but more significantly for CO hydrogenation. For example, at 533 K the methanol synthesis activity of 30.5 wt% zirconia-promoted Cu/SiO2 is 84 and 25 times that of unpromoted Cu/SiO2 for CO and CO2 hydrogenation, respectively. For all catalysts, the rate of methanol synthesis from CO2/H-2 is higher than that from CO/H-2. The apparent activation energy for methanol synthesis from CO decreases from 22.5 to 17.5 kcal/mol with zirconia addition, suggesting that zirconia alters the reaction pathway. For CO2 hydrogenation, the apparent activation energies (similar to 12 kcal/mol) for methanol synthesis and the reverse water-gas shift (RWGS) reaction are not significantly affected by zirconia addition. While zirconia addition greatly increases the methanol synthesis rate for CO2 hydrogenation, the effect on the RWGS reaction activity is comparatively small. The observed effects of zirconia are interpreted in terms of a mechanism, in which zirconia serves to adsorb either CO or CO2, whereas Cu serves to adsorb H-2. It is proposed that methanol is formed by the hydrogenation of the species adsorbed on zirconia.