The flow distribution in fuel cells has an important influence on both the power density and efficiency of fuel cell systems. In order to effectively utilize the area, flow distribution should be as homogeneous as possible. In addition, pressure losses should be minimized with regard to the power demand of auxiliary components as pumps and compressors. In polymer electrolyte fuel cells (PEFCs) and direct methanol fuel cells (DMFCs) the flow field is in direct contact with the diffusion layer. The main task of the diffusion layer is to distribute the reactants from the flow field towards the catalyst layer. To prevent diffusion overvoltages, the diffusion layer is in general highly porous and provides high fluxes of the reactants. Consequently, the flow distribution in the flow field can be superpositioned by a flow in the diffusion layer. In this paper, we discuss the interaction between the diffusion layer and the flow field. Experimentally, we characterized different diffusion layers with regard to their diffusion properties as well as different flow fields. Additional simulation studies help to understand the processes and to determine suitable combinations of flow fields and diffusion layers. (C) 2003 Elsevier B.V. All rights reserved.