Cu-Ce/gamma-Al2O3 catalysts promoted with cobalt are tested for the low-temperature selective oxidation of carbon monoxide (CO) in excess hydrogen, as produced by a fuel processor for proton exchange membrane fuel cells (PEMFCs). A small addition (0.2 wt.%) of Co to Cu-Ce/gamma-Al2O3 leads to a large increase in the activity for selective CO oxidation. When either CO2 (13 vol.%) or H2O (10 vol.%) is present in the reformed gas feed, both Cu-Ce/gamma-Al2O3 and Cu-Ce-Co/gamma-Al2O3 show a decrease in CO oxidation activity at low temperatures, especially, under 200degreesC. Compared with Cu-Ce/gamma-Al2O3, however, Cu-Ce-Co/gamma-Al2O3 gives higher resistance to CO2 and H2O. There also exists a temperature window at 210-225degreesC that corresponds to the conversion of 99.9% CO. From stability tests and temperature-programmed (TPD) desorption studies of CO2/H2O, it is concluded that the main cause for the decrease in catalytic activity with CO2 and H2O in the feed is due to competitive adsorption of CO and CO2 as well as to the blockage of the active sites by water vapor at low reaction temperatures. (C) 2004 Published by Elsevier B.V.