The hydrogenation of nitrates in aqueous solution has been studied using Pd-Cu catalysts deposited in mesoporous ceramic membranes operated in a flow-through catalytic membrane reactor. The effect of the top-layer pore size (5, 10 or 25 nm) on the catalytic activity was explored. The activity increased with trans-membrane flow rate for the three membranes, although moderately with the 25 nm pore membrane. At a similar flow rate, the activity increased when the pore size decreased. Concentration polarization of nitrates was evidenced by ultrafiltration experiments with bare membranes, and was enhanced when the pore size decreased. Concentration polarization appeared effective in improving the catalytic activity, due to a local increase in nitrate concentration in the catalytic top-layer of the membranes. (C) 2010 Elsevier B.V. All rights reserved.