The catalytic behavior of Cu catalysts supported on ultrapure silica and promoted with Ca, Zn, and La oxides was investigated in the hydrogenation of CO and CO2 to methanol at high pressure. Cu on very pure silica produces hardly any methanol, while the addition of basic oxides and the use of gamma-alumina as support improve the catalyst performance. The strong promoting effect of Ca and La oxide on the silica-supported Cu and the weak promoting effect for alumina-supported Cu suggest that the basic oxide additives must be close to or in contact with the Cu particles to be effective in methanol synthesis. The methanol activity of Zn/Cu/SiO2 increased with increasing CO2 content in a CO-CO2-H-2 mixture, suggesting that CO2 is the main carbon source for methanol.