The catalytic activity of Cu(100) and Ni/Cu(100) with respect to the methanol synthesis from various mixtures containing CO2, CO, and H-2 have been studied in a combined UHV/high pressure cell apparatus at reaction conditions, P-tot = 1.5 bar and T= 543 K. For the clean Cu(100) surface it is found that admission of CO to a reaction mixture containing CO2 and H-2 does not lead to an increase in the rate of methanol formation, which indirectly suggests that the role of CO in the industrial methanol process relates to the change in reduction potential of the synthesis gas. For the Ni/Cu(100) surface it is found that Ni does not promote the rate of methanol formation from mixtures containing CO2 and H-2. In opposition, admission of CO to the reaction mixture leads to a significant increase in the rate of methanol formation with a turnover frequency/Ni site similar to 60 x the turnover frequency/Cu site at Ni coverages below 0.1 ML making it a rather substantial promoting effect. It is found that the admission of CO to the synthesis gas creates segregation of Ni to the surface, whereas this is not the case for a reaction involving CO2 and H-2. It is suggested that CO acts strictly as a promotor in the system and we ascribe the increase in activity to a promotion through gas phase induced surface segregation of Ni.