The organically treated montmorillonite (MMT) clays incorporated into the Nafion matrix by means of solution intercalation using dimethylacetamide (DMA) as a solvent have been tested. The diffraction patterns of the nanocomposite Nafion/MMT membranes indicated a disordered and exfoliated nanocomposite structure. The transmission electron microscopy images revealed that the clay layers are well nanodispersed in the Nafion matrix until the weight fraction of the MMT reaches 10 wt %. The apparent improvement in thermal stability might result from the strong interaction between Nafion and MMT. The refractive index measurements indicated that the methanol permeability of the Nafion/MMT ultrathin membrane containing 1 wt % MMT was 14 times lower in comparison to commercial Nafion 117 membranes. The direct methanol fuel cell (DMFC) with polytetrafluoroethylene-reinforced Nafion/MMT nanocomposite membrane (25 mu m) at 400 mA/cm(2) under dry air conditions and 60 degrees C cell temperature demonstrated 120 mW/cm(2) power output, which was 50% higher in comparison to the DMFC with commercial Nafion 117 membrane. (c) 2006 The Electrochemical Society.