The effects of co-catalyst were studied on the methanol-synthesis reaction from carbon dioxide and hydrogen over various TiO2-supported copper catalysts. The catalytic activities with a continuous flow reactor were measured together with in situ infrared spectra at the elevated temperature and pressure. Phosphorus did not show any activity. Potassium showed a high activity but promoted CO formation. ZnO had a high activity and a high methanol selectivity. ZrO2 also showed promotion effect to some extent. By in situ Fourier transform infrared spectroscopy (FTIR), the formate species formed on Cu was observed over CuO/TiO2 catalyst. The amount of the formate species depended on the kinds of additive. Addition of K, Zr and Zn showed high activity and increased the intensities of the formate species. On the other hand, addition of P decreased the activity and the intensities of the formate species. Among them, Zn addition increased the amount of adsorbed formate species markedly and obtained the highest methanol yield.