The performance of PEM fuel cells depends significantly on an appropriate flow field design. In this work, an investigation on the influence of channel geometries on stack performance has been carried out. A parallel flow field design was developed to widely eliminate the influence of other flow field design parameters, e.g. degree of parallelization and gas crossover effects at the turning points of the gas channels. Starting from a basic design, channel as well as rib dimensions were varied and their influence on stack performance were studied. An optimum between 0.7 and 1 mm was found for either channel or for rib widths. For wider dimensions, the influence on mass transport (ribs) or lateral conductivity (channels) has been found to become significant. For very small dimensions, the manufacturing effort becomes unacceptably high (ribs) and the probability of channel clogging by formation of water droplets increases. In general, narrow channel dimensions are preferred for high current densities, whereas wider dimensions are better at low current densities. (c) 2005 Elsevier B.V. All rights reserved.