The carbon monoxide (CO) poisoning effect on carbon supported catalysts (Pt-Ru/C and Pt/C in polymer electrolyte membrane (PEM) fuel cells has been investigated at higher temperatures (T > 100 degrees C) under different relative humidity (RH) conditions. To reduce the IR losses in higher temperature/lower relative humidity, Nafion-(R) Tetlon-(R) Zr(HPO4)(2) composite membranes were applied as the cell electrolytes. Fuel cell polarization investigation as well as CO stripping voltammetry measurements was carried out at three cell temperatures (80, 105 and 120 degrees C) with various inlet anode relative humidity (35%, 58% and 100%). CO concentrations in hydrogen varied from 10 ppm to 2%. The fuel cell performance loss due to CO poisoning was significantly alleviated at higher temperature/lower RH due to the lower CO adsorption coverage on the catalytic sites, in spite that the anode catalyst utilization was lower at such conditions due to higher ionic resistance in the electrode. Increasing the anode inlet relative humidity at the higher temperature also alleviated the fuel cell performance losses, which could be attributed to the combination effects of suppressing CO adsorption, increasing anode catalyst utilization and favoring OHads group generation for easier CO oxidation. (c) 2006 Elsevier Ltd. All rights reserved.