Nafion(R)/montmorillonite (MMT) nanocomposite membranes were prepared to reduce methanol crossover while maintaining essential proton conductivity. When the loading of MMT clay was less than 20 wt.%, the X-ray diffraction patterns of Nafion((R))/MMT nanocomposite membranes were featureless, indicative of the disordered and exfoliated structure of clay. It was observed that both tensile strength and elongation at break were improved by the nanodispersed clay particles. In addition, the thermal decomposition temperature of Nafion((R)) backbones shifted to higher temperature region due to the strong interaction of polymer main chains with organo-clays. Refractive index (RI) measurement showed that methanol permeability of unmodified Nafion((R)) was ca. 2.3 x 10(-6) cm(3) cm/cm(2) s while that of 1 wt.%((R)) Nafione((R))/MMT nanocomposite membrane decreased down to 1.6 x 10(-7) cm(3) cm/cm(2) s without apparent conductivity loss. As a result, 1 wt.% Nafion((R))/MMT nanocomposite membranes delivered much higher power density than unmodified Nafion((R)) membrane at concentrated methanol feed up to 10 M. (C) 2004 Elsevier Ltd. All rights reserved.