Small angle neutron scattering experiments were performed on a proton exchange membrane fuel cell in order to determine the transversal water concentration profile, within the polymer electrolyte. This is determined under state-of-art operating conditions. From the scattering spectra, different concentration profiles were deduced and discussed in terms of a competition between electro-osmosis and back diffusion. For steady states, the balance between these driving forces is modified by the current density. It is also demonstrated that this equilibrium is laterally nonuniform in the fuel cell. This is especially pronounced between the gas inlets and outlets, in a co-flow geometry. (c) 2007 The Electrochemical Society.