We report the synthesis of nafion-zeolite (Fe-silicalite-1) nanocomposite membranes using either a supercritical CO2 pre-treatment followed by impregnation with zeolite nanoparticles or in situ synthesis of Fe-silicalite-1 inside the pores of nafion membrane. The effects of the supercritical CO2 activation and the size of embedded zeolite particles on the proton conductivity and methanol permeability of composite membranes are observed. In selected cases, the membranes exhibit extremely low methanol permeability and high selectivity (defined as proton conductivity/methanol permeability ratio). (c) 2007 Elsevier B.V. All rights reserved.