Methanol production from CO/CO2/H-2 over a Cu/ZnO/Al2O3 catalyst has been compared in an internal recycle reactor (finite conversion) and a differentially operated tubular reactor. From experiments at differential conversion it was found that the intrinsic rate of CO2 hydrogenation was approximately 20 times faster than the intrinsic rate of CO hydrogenation. The rate of methanol production at differential conversion increased almost linearly with respect to the CO2 concentration, indicating that there is negligible inhibition by CO2 in the absence of products. With 1-4% CO2 in the synthesis gas, the methanol production rate was found to be substantially greater at finite conversion than at differential conversion, which is evidence for a promoting influence by the product water, With >4% CO2 in the synthesis gas there was increasing loss of methanol production at finite conversion, and under CO2/H-2 in particular there was an order of magnitude difference between methanol production at finite and differential conversions, This effect was due to inhibition of CO2 hydrogenation associated with the presence of product water.