The physico-chemical origins of the performance loss in polymer electrolyte fuel cells operated at high current densities are investigated. This effect could be attributed to a shortage of water in the proton conducting membrane leading to an increase of the membrane proton-conduction resistance as well as to a increase of the activation overpotential for the hydrogen oxidation reaction due to a decrease in the number of active sites on the anode side. To fortify this thesis, we present measurements of the fuel cell impedance, varying the membrane thickness, the ionic density, and the humidification conditions of the reactants.