Acidic and neutral Nation (R) 115 perfluorosulphonate membranes have been modified by in situ polymerization of pyrrole using Fe(III) and H2O2 as oxidizing agents, in order to decrease methanol crossover in direct methanol fuel cells. Improved selectivities for proton over methanol transport and improved fuel cell performances were only obtained with membranes that were modified while in the acid form. Use of Fe(III) as the oxidizing agent can produce a large decrease in methanol crossover, but causes polypyrrole deposition on the surface of the membrane. This increases the resistance of the membrane, and leads to poor fuel cell performances due to poor bonding with the electrodes. Surface polypyrrole deposition can be minimized, and surface polypyrrole can be removed, by using H2O2. The use of Nation in its tetrabutylammonium form leads to very low methanol permeabilities, and appears to offer potential for manipulating the location of polypyrrole within the Nafion structure. (c) 2005 Elsevier Ltd. All rights reserved.