Commercial sulfonated fluorocarbon membranes (Nafion) have been impregnated with poly(1-methylpyrrole) by in situ polymerization. This can lead to a decrease of more than 90% in the permeability of the membranes to methanol, although the ionic resistance of such heavily loaded membranes becomes too high for high power fuel cells. At lower poly(1-methylpyrrole) loadings, a decrease in methanol permeability by as much as 50% can be realized without a significant increase in ionic resistance. Such modified membranes can provide comparable performances to the original membranes in hydrogen fuel cells.