This work has been focused on the preparation and characterization of composite membranes with thickness between 19 and 97 mu m and containing Nafion(R) infiltrated into a porous mat obtained by electrospinning of an aqueous solution of Poly vinyl alcohol (PVA). The mat was composed of PVA nanofibers with diameters between 200 and 300 nm, which were functionalized on their external surface with sulfonic acid groups in order to cooperate in the proton conductivity of Nafione(R). The proton conductivity of a composite membrane (47 mu m thick) measured by impedance spectroscopy reached 0.022 S/cm at 70 degrees C and fully hydrated. This value is lower than the conductivity measured in the same conditions for a pristine Nafion(R) cast membrane (46 mu m), 0.032 S/cm. However, the performance of both membranes in Direct Methanol Fuel Cell tests was evaluated and showed comparable results. The proton conductivity of a series of cast Nafion(R) membranes with thickness similar to those of the composite membranes was found to behave linearly thickness-dependent, while NAF/PVA composite membranes do not show such a linear behavior due to their heterogeneous composition. Nevertheless, the composite membrane presenting a thickness of 47 mu m records the maximum peak conductivities at the whole temperature range. An intrinsic value for the activation energy of our cast Nafion(R) membranes at fully hydrated conditions was estimated to be 7 kJ/mol. (C) 2011 Elsevier B.V. All rights reserved.