The preparation and characterization of new polymer composite membranes containing polyphenylene oxide (PPO) thin films with phosphomolybdic acid (PMA) are presented. PPO thin films with PMA (PPO-PMA films) have been prepared by using the solvent mixture. In this study, methanol was used as a solvent dissolving PMA and chloroform was used as a solvent dissolving PPO. PPO-PMA solutions were cast onto a glass plate. The composite membranes were prepared by casting Nafion mixture on porous PPO-PMA films. The composite membranes were characterized by measuring their ion conductivity and methanol permeability. The morphology and structure of these PPO-PMA films were observed with scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and wave dispersive spectrometer (WDS). The composite membranes are tested as electrolytes in a direct methanol fuel cells (DMFCs). The performance tests have shown that they have a good perspective in this DMFCs. The methanol cross-over in the DMFC can be reduced by composite membranes containing PPO-PMA barrier films. (c) 2004 Elsevier B.V. All rights reserved.