Nafion/polypyrrole composite membrane was prepared by chemical in situ polymerization for a direct methanol fuel cell. The composite membrane was characterized by quantitative analysis, DMA, TGA, EPMA, water and methanol uptake tests, conductivity and permeability measurements, and unit cell operation. The mechanical and thermal properties of the composite membrane were enhanced due to the interaction between the polar phase of Nation and secondary ammonium groups of polypyrrole. In addition, the sorption properties of the composite membrane influenced two transport properties, proton conductivity and methanol crossover. The optimization of cell performance was related to the distribution of polypyrrole particles over the cross section of the membrane. In particular when the polypyrrole particles were mainly present near the surface rather than in the internal space, the difference in the relative proton conductivity and the relative methanol permeability was largest. It is supposed that the existence of the polypyrrole particles near the surface such as thin film was favorable to reducing methanol crossover with maintaining proton conductivity. As a result of that, N/P 003 had higher performance than Nation under the specific condition. (c) 2005 Elsevier B.V. All rights reserved.