Effects of hydrophobic polymer content within a carbon paper, used as the cathode gas diffusion layer (GDL), on power performance of a H-2/air proton exchange membrane fuel cell (PEMFC) have been studied. Electrochemical methods are used in conjunction with morphology and wetting property characterization. Surface contact angle of wet-proof-treated GDL as a function of temperature is measured by a novel capillary rise method. It is shown that the contact angle generally decreases with the temperature, and that there is insignificant difference in contact angle on carbon papers treated with different contents of fluorinated ethylene propylene (FEP) ranging from 10 to 40 wt.%. Under all humidification conditions in this study, a membrane-electrode assembly (MEA) consisting of 10 wt.% FEP-impregnated GDL shows higher power densities than 30 wt.% FEP-impregnated one. Surface morphology of the hydrophobic polymer-treated carbon paper has been analyzed by scanning electron microscopy (SEM) and is identified as playing a crucial role in affecting the power performance of such treated GDL in the PEM fuel cell. (C) 2004 Elsevier Ltd. All rights reserved.