The effects of adding mixtures of titania and zirconia on the methanol synthesis activity and selectivity of Cu/SiO2 were investigated. The synthesis of methanol from both CO/H-2 and CO2/H-2 mixtures was examined at 0.65 MPa and temperatures between 448 and 573 K. For CO hydrogenation, the addition of ZrO2 alone increased the methanol synthesis activity of Cu/SiO2 by up to three-fold. Substitution of a portion of the ZrO2 by TiO2 decreased the methanol synthesis activity of the catalyst relative to that observed when only ZrO2 is added. ZrO2 addition also enhanced the methane synthesis activity by as much as seven-fold. In the case of CO2 hydrogenation, the maximum methanol synthesis activity is achieved when a 50/50 wt% mixture of ZrO2 and TiO2 is added to Cu/SiO2. Neither the presence of the oxide additive nor its composition had any effect on the activity of the reverse water-gas-shift reaction, which suggests that this reaction proceeds only on Cu. The observed effects of ZrO2 and TiO2 on the catalytic activity of methanol synthesis from CO and CO2, and methane synthesis from CO, are interpreted in terms of the strength and concentration of acidic and basic groups on the surface of the dispersed oxide.