The objective of this study is to determine the effects of various factors on the performance of proton exchange membrane (PEM) fuel cell. These factors are membrane thickness, hot-pressing conditions of the gas diffusion layer (GDL) either onto the membrane or membrane electrode assembly (MEA) and Teflon:carbon ratio in the GDL on PEM fuel cell performance. Homemade five-layer and commercial three-layer MEAs were used in the experiments. Nafion (R) 112 and 115 which have nominal thicknesses of 50 and 125 mu m, respectively, were used as membranes. It was observed that fuel cell performance is inversely proportional to membrane thickness. In the case of five-layer MEAs, optimum hot-pressing conditions of catalyst-coated GDLs onto the membrane were found as 172 N cm(-2). However, the maximum performance for three-layer MEAs was obtained with no press conditions. Also, by increasing Teflon:carbon ratio in the GDLs, PEM fuel cell performance increases up to a certain value, but further increase of this ratio worsen the performance. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.