The possibility of ultrathin Nafion/expanded polytetrafluoroethylene (ePTFE) membranes used as proton-exchange membranes (PEMs) for direct methanol fuel cells (DMFCs) was investigated in this study. Nafion/ePTFE membranes with a thickness of similar to 14 mu m were promoted by self-assembling Pd nanoparticles on the surface to reduce the methanol crossover. The loading of the Pd nanoparticles assembled on the membranes was 1.6-1.8 mu g/cm(2) and had little effect on the high conductivity of the Nation membranes. With the self-assembly of Pd nanoparticles, the methanol permeation noticeably decreased from 340 to 28 mA/cm(2). As a result, the open-circuit voltage of the Nafion/ePTFE membranes that were self-assembled for 48 h had a more significant increase from 0.55 to 0.73 V. The reduction of methanol crossover significantly increased the DMFC voltage-current performance, and this means that self-assembled Nafion/polytetrafluoroethylene PEMs have promise in DMFCs. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 2227-2233, 2008