A twin-cell electrochemical filter is demonstrated to reduce the CO concentration in reformate hydrogen. In this design, the potential and gas flow are switched between the two filter cells so that alternative CO adsorption and oxidation occur in each cell while providing a continuous flow of H-2 to a fuel cell. The effects of filter switching time and applied potential on the CO concentration of gas exiting the filter are presented here for a CO concentration of 1000 ppm in nitrogen flowing at 100 cm(3)/min. The parasitic loss of hydrogen from a corresponding reformate stream was estimated to be 1.5%. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3267850] All rights reserved.