Perfluorosulfonate ionomer dispersions in dimethylformamide solvent were used to form solution-cast membranes. Modified composite membranes were prepared with the addition of 1 and 5 wt.% montmorillonite salts. Measurements of water uptake, solubility and methanol permeation of the cast membranes were conducted. Tangential conductivities were measured directly on membranes fully immersed in deionized water by means of impedance spectroscopy. Results show that the addition of a low quantity of silicate did not alter the conductivity (94-96 mS cm(-1) at 25 degrees C, but produced a marked decrease of methanol permeability (-6%). Also a simple model was proposed to explain the increase of tangential conductivity with decreasing thickness of Nafion (R) and recast membranes. (c) 2004 Elsevier Ltd. All rights reserved.