We modified Nafion by means of chemical in situ polymerization of pyrrole monomers with platinum (Pt) precursors for an application into an electrolyte of direct methanol fuel cells (DMFCs). SEM and EPMA exhibited the presences of polypyrrole and Pt at the surface region of Nafion, after diffusing and polymerizing pyrrole monomers with Pt precursors. XPS and FT-IR spectra were used to characterize the surface of Naf-Ppy-Pt composite membranes, demonstrating that pyrrolinum groups of polypyrrole were interacted with sulfonic groups or Pt precursors (PtCl6- or PtCl4-). After in situ polymerization of pyrrole monomers, the morphological reorganization of sulfonic groups in Naf-Ppy-Pt composite membranes occurred via electrostatic interaction. Thermal stability, proton conductivity, methanol permeability, and cell performance of composite membranes were analyzed by TGA, AC impedance, refractometer, and potentiostat. Naf-Ppy-Pt composite membranes had higher thermal stabilities of sulfonic groups and side chains than Nation and Naf-Ppy as a result of the interaction between Nafion-SO3-center dot center dot center dot polypyrrole-NH2+ and the presence of thermally stable Pt., The cell performance of Naf-Ppy-Pt 0 0 2 was enhanced significantly compared to that of Nation under the specific condition, due to more reduction of methanol crossover than that of proton conductivity. Therefore, this synthetic method offers a facile way to improve physical properties of polymer electrolyte for the fabrication of advanced composite membranes. (C) 2007 Elsevier B.V. All rights reserved.