The methanol and water crossover through the polymer electrolyte membrane (PEM) of the direct methanol fuel cell (DMFC) is analysed experimentally and theoretically. Crossover fluxes have been measured for different operating conditions using a miniplant with full online material balancing of all DMFC inlet and outlet material flows. The experimental data are compared to simulation results obtained from a one dimensional rigorous mathematical model of the DMFC. In this model, the main focus lies on the realistic description of mass and energy transport and physical properties of the PEM material. For mass transport, a model based on the generalised Maxwell-Stefan equations is formulated. To account for the very complex PEM behaviour (swelling with water/methanol as well as partition equilibria for both components between free liquid/gas phase and pores within the PEM), a Flory-Huggins activity model is used for the mobile species inside the PEM. The parameters of this model are determined from experiments. Membrane swelling is accounted for by an adaptive spatial discretisation in the model implementation. (c) 2005 Elsevier B.V. All rights reserved.