The mass-transfer phenomena at the electrodes and, in particular, the diffusion of oxygen at the cathode significantly affect the limit performance of PEMFCs. Some particular geometric arrangements, such as the interdigitated or serpentine, have demonstrated their effectiveness in lowering diffusive resistances. It is possible to have a better understanding of these phenomena by determining the various possible diffusive regimes taking place inside the porous layer close to the electrodes. In each regime the interaction between diffusive and forced flows can be expressed in terms of Peclet numbers and the overall diffusive resistance in terms of Sherwood numbers. In this way, the comparison of traditional and non-traditional geometric arrangements can be more deeply studied, so that the problems relating to the simulation and optimisation of the cell can be more efficiently dealt with. (c) 2005 Elsevier B.V. All rights reserved.