We have studied the rate of methanol formation over Cu(100) and Ni/Cu(100) from various mixtures of CO, CO2 and H-2. It is found that the presence of submonolayer quantities of Ni leads to a strong increase in the rate of methanol formation from mixtures containing all three components whereas Ni does not influence the rate from mixtures of CO2/H-2 and CO/H-2, respectively. The influence of the partial pressures of CO and CO2 on the rate indicates that the role of CO is strictly promoting. From temperature-programmed desorption spectra it follows that the surface concentration of Ni depends strongly on the partial pressure of CO. In this way the increase in reactivity is interpreted as a CO-induced structural promotion introduced by the stronger bonding of CO to Ni as compared to Cu. It is suggested that this type of promotional behavior will be of general importance in existent catalysts and perhaps even more relevant in the development of new or improved bimetallic catalysts.