Commercial perfluorosulphonic acid membranes (Nafion) have been impregnated with polypyrrole by in situ polymerization to decrease the crossover of methanol in direct methanol fuel cells (DMFCs). Modified membranes produced by polymerization of the pyrrole with hydrogen peroxide and iron(III) have been evaluated in a DMFC. Both methods produce membranes that can provide enhanced cell performance, although membranes produced with iron(III) as the oxidizing agent for the polymerization require additional treatments to restore their conductivity and promote bonding to the electrodes. Performance gains result from substantial reductions of the cathode overpotential, while anode overpotentials increase due to the lower conductivities of the modified membranes. Part of the beneficial effect at the cathode appears to be due to lower water crossover from the anode to the cathode. (C) 2003 The Electrochemical Society.