The loss of Cu/ZnO/Al2O3 activity for methanol synthesis has been studied over 50 h under a range of synthesis gas compositions. Experiments have been carried out at differential and finite conversions in order to compare deactivation under the pure feed to that in the presence of products. Under differential conversion, it was found that the degree of deactivation was related to the CO concentration, being particularly acute under CO/H-2. Under CO/H-2, there was a close correlation beta een activity loss and loss of Cu surface area, suggesting that an important cause of deactivation is sintering, which may be promoted by over-reduction of Cu. At finite conversion with CO/H-2 feed the deactivation was less severe. This lower rate of deactivation is most likely due to exposure of the catalyst to products that can stabilize oxidized Cu species. Interestingly, under CO2/H-2, the deactivation was negligible at differential conversion. However some deactivation was apparent at finite conversion. The exposure of the catalyst to the products, CO and water, at finite conversion can explain these results.