The temperature and concentration dependent effects of CO and CO2 on the performance of a similar to 3 mu m thick Pd/Ag 23 wt% membrane, employed in a microchannel configuration, were investigated. The microchannel system consisted of six parallel channels, 13 mm long, 1 mm wide and 1 mm deep. The membrane permeance was determined to 5.1 X 10(-3) Mol m(-1) s(-1) Pa-0.5 at 300 degrees C under pure hydrogen. After the last experiment, a small leakage occurred, that reduced the H-2/N-2 separation factor to similar to 3300 at 200 kPa absolute pressure difference. Both CO and CO2 showed an inhibitive effect on hydrogen permeation. The CO effect was strongly dependent on both temperature (275-350 degrees C) and CO concentration/partial pressure (0-5 mol%). The CO inhibition occurred rapidly upon exposure, with a sharp drop in flux between 0 and 0.25 mol% CO. The time required to restore the initial flux value after CO exposure became longer when the exposure temperature was lowered. CO desorption hence was the main mechanism for flux restoration at the higher temperatures, while it was controlled by other, slower processes at the lower temperatures. The effect Of CO2 was slower, and long time exposure was necessary to reach apparently stable values. Only a weak effect was observed at 350 degrees C, while at 300 degrees C, a nearly linear decrease was observed over several days. We suggest that the main inhibition mechanism was not CO2 (or CO from reverse WGS) competitive adsorption, but rather a slow formation and removal of strongly adsorbed species. (C) 2009 Elsevier B.V. All rights reserved.