The effectiveness of a commercial activated carbon modified by KOH (KMAC) was evaluated as adsorbent for purifying NOx and SO2, which are the major contaminants in fuel cell cathode air stream. The N-2 adsorption-desorption isotherms of KMAC samples showed that the surface structure of the activated carbon was changed significantly by KOH impregnation. The sample of KMAC with a loading of 10.1% KOH by weight presented the highest adsorption capacities for both NOx and SO2, which were 96 mg g(-1) and 255 mg g(-1), respectively. A pre-exposure of KMAC to CO2 caused neither effect on the adsorption of NOx nor on the adsorption of SO2. KMAC could fully protect a 250 W proton exchange membrane fuel cell (PEMFC) stack from 1100 ppb of NOx and 250 ppb of SO2 for about 130 h. (C) 2007 Elsevier B.V. All rights reserved.