This work summarizes the results obtained from the modification of perfluorinated sulfocationic MF-4SC membranes by in situ polymerization of aniline. The transport properties of polyaniline/MF-4SC composite membranes after different times of polymerization have been analyzed. It is observed that after 3 h of aniline polymerization, composite membranes exhibit higher protonic conductivity compared to the pristine sample. The performance of fuel cells containing membrane electrode assemblies (MEAs) from MF-4SC and polyaniline/MF-4SC has also been studied. The kinetic parameters governing the voltage dependence on current density have been estimated and the results have been fitted to a previously established one-dimensional isothermal model by Springer et al. [J. Electrochem. Soc. 138, 2334 (1991)]. Good agreement between experimental data and model has been found for low density currents. Some insights about the membrane proton conductivity mechanism are given. (C) 2010 The Electrochemical Society. [DOI:10.1149/1.3439671] All rights reserved.