We conducted the WGS reaction on a catalytic membrane reactor consisting of a WGS catalyst bed, Pt/CeO2 and thin, defect-free, Pd-Cu alloy membranes. The presence of CO and other gases with H2 reduced the H2 permeation through the membrane by more than 50% and the effect of the other gases on the permeation reduction decreased in the following order: CO>CO2>N2. In a catalytic membrane reactor with helium sweep gas, the CO conversion was improved by about 65% compared with the catalyst without any membrane, and the CH4 formed from an undesirable side reaction was significantly reduced. Although the H2 permeation was severely reduced by surface phenomena such as blocking of available H2 dissociation sites by CO, CO2 and steam, the CO conversion was notably improved by the membrane presence. Moreover, the CO conversion was maintained at 98% even after 60 h of reaction and our Pd-Cu-Ni alloy membrane withstood the exposure of CO and the other gases. However, for separation of pure H2, a newly designed, catalyst-membrane system is required with better sealing and the ability to withstand the high operating pressure that drives the H2 permeation.