The low temperature (403-453 K) conversions of CO/hydrogen and CO2/hydrogen mixtures (6 bar total pressure) to methanol over copper catalysts are both assisted by the presence of small amounts of water (mole fraction similar to 0.04-0.5%). For CO2/hydrogen reaction mixtures, the water product from both methanol synthesis and reverse water-gas shift serves to initiate both reactions in an autocatalytic manner. In the case of CO/D-2 mixtures, very little methanol is produced until small amounts of water are added. The effect of water on methanol production is more immediate than in CO2/D-2, yet the steady-state rates are similar. Tracer experiments in (CO)-C-13/(CO2)-C-12/hydrogen (with or without added water) show that the dominant source of C in the methanol product gradually shifts from CO2 to CO as the temperature is lowered. Cu-bound formate, the major IR visible surface species under CO2/hydrogen, is not visible in CO/moist hydrogen. Though formate is visible in the tracer experiments, the symmetric stretch is absent. These results, in conjunction with recent DFT calculations on Cu(111), point to carboxyl as a common intermediate for both methanol synthesis and reverse water-gas shift, with formate playing a spectator co-adsorbate role. (c) 2012 Elsevier Inc. All rights reserved.