Studies of the CO tolerance for the hydrogen oxidation reaction (hor) on a polymer electrolyte fuel cell were conducted on modified gas diffusion electrodes with CO filtering layers formed by ruthenium dispersed onto carbon (Ru/C) and having platinum dispersed on carbon (Pt/C) in the catalyst layer. Different configurations for gas diffusion electrodes containing Ru hydrated-oxide (RuOxHy/C) layers were tested in order to understand the CO oxidation mechanism on these catalysts. Single cell polarization results indicated that a direct contact between Ru distributed on the diffusion layer and the catalyst layer is required to achieve CO tolerance. Results also show that an enhanced activity is achieved for the hor in the presence of CO with PtRu/C in the catalyst layer obtained by an impregnation method. Also, an electrode formed by a RuOxHy/C filtering layer in contact with a catalyst layer with PtRu/C showed the highest CO-tolerance. For this system, the cell voltage loss was only 65 mV at 1 A cm(-2) with H-2 divided by100 ppm CO, with respect to the value obtained with pure hydrogen. (C) 2004 Elsevier B.V. All rights reserved.