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Commercial perfluorosulphonic acid membranes ͑Nafion͒ have been impregnated with polypyrrole by in situ polymerization to decrease the crossover of methanol in direct methanol fuel cells ͑DMFCs͒. Modified membranes produced by polymerization of the pyrrole with hydrogen peroxide and iron͑III͒ have been evaluated in a DMFC. Both methods produce membranes that can provide enhanced cell performance, although membranes produced with iron͑III͒ 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.Direct methanol proton exchange membrane fuel cells ͑DMFCs͒ 1-3 are currently being developed by many companies for commercialization over the next few years. 4,5 Initial applications will be in consumer electronics such as cell phones and computers, while transportation and utility applications will become increasingly important toward the end of this decade.Organizations that are involved in the development of the DMFC face a number of significant obstacles that currently limit the potential market. 4 The crossover of methanol through the membrane from the anode to the cathode is a major problem. 3 It decreases both the performance and the fuel efficiency of the cell, and limits its lifetime.Conjugated polymer/Nafion composite membranes show excellent potential for DMFC applications 6,7 because they are less permeable to methanol and therefore efficiency losses from methanol crossover are significantly less. Preliminary results have demonstrated that composite membranes prepared using poly͑1-methylpyrrole͒ reduce methanol crossover by as much as 50% without a significant increase in the resistance of the composite membrane. 6 The work described here characterizes the properties of polypyrrole/Nafion composite membranes in an operational DMFC. Pyrrole was selected as the polymer precursor in this work because its greater solubility in water simplifies the membrane modification process and preliminary results in fuel cells were better than those achieved with a poly͑1-methylpyrrole͒/Nafion composite. Methanol crossover and DMFC performance ͑including the separate performances of the anode and cathode͒ have been evaluated and results are compared with results obtained with unmodified Nafion membranes. ExperimentalMembrane modification.-Membranes were cleaned in 15% H 2 O 2 (aq), 1 M H 2 SO 4 (aq) ͑1 h at 60-80°C in each solution͒, and water. They were then immersed in a pyrrole solution ͑0.08-0.10 M for 80-120 min for Fe͑III͒ as the oxidant ͑except F5E͒, 0.20 M for 5 min ͑15 min for H5B͒ for H 2 O 2 ), and then rinsed well with water. The pyrrole within the membrane was then polymerized by immersion in a 0.03-0.05 M Fe͑NO 3 ) 3 (aq) solution for 8...