In this work the water gas shift (WGS) reaction in a membrane reactor (MR) using a porous stainless steel supported silica membrane and a CuO/CeO2 based commercial catalyst was analysed in a temperature range of 220-290 degrees C up to 600 kPa. The reaction pressure effect on the CO conversion and permeate stream composition was followed with special attention paid to the significant performance improvement in the MR. The best operating condition for the MR was identified as 280 degrees C and 400 kPa, obtaining a CO conversion of 95% with an increment of 8% with respect to TR. Furthermore, the membrane permeance (H-2 : 9.7-29; CO : 0.3-1. 1; CO2: 0.4-1.5 nmol/m(2)s Pa) and selectivity (H-2/CO, H-2/CO2 ranging from 15 to 40) and the influence of other gases on H-2 permeation were evaluated before and after reaction testing. No inhibition effect of other gases on the hydrogen flux in all cases was observed. An MR using silica membranes was successfully employed in the WGS reaction, obtaining always higher conversion than with a traditional reactor (TR) and also (at T > 250 degrees C) exceeding the TR equilibrium conversion (TREC). (c) 2007 Elsevier Ltd. All rights reserved.