Proton exchange membrane fuel cells (PEMFCs) most likely will use reformed fuel as the primary source for the anode feed which always contains carbon dioxide (CO) and hydrogen sulfide (H2S). Trace amount of CO and H2S can cause considerable cell performance losses. A comparison between the effect of CO and that of H2S on PEMFC performance was made in this paper. Under the same conditions, the H2S poisoning rate is much higher than CO because of different adsorption intensity. When the fuel stream contains the gas mixture (25 ppm CO and 25 ppm H2S), the fuel cell performance deteriorates more quickly than 50ppm CO but slowly than 50ppm H2S and can be only partially recovered by reintroducing neat H-2. The resulting effects of the mixtures can be divided into two parts roughly: during the inception phase, the cell voltage drops quickly and the actual values of anode overvoltage are bigger than the corresponding calculated values; then the deterioration rate of the cell performance decreases gradually. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.