A freestanding polymer electrolyte membrane was developed having low permeability to methanol while maintaining high permeability to water and proton conductivity. A two-polymer composite formed an interpenetrating polymer network (IPN) composed of proton-conducting 2-acrylamido-2-methyl propanesulfonic acid and a second polymer, poly(vinyl alcohol), serving as a methanol barrier. Ion conductivity and methanol permeability were controlled by adjusting polymer ratios and the extent of cross-linking of the two polymers. Compared to Nafion, the IPNs had up to fifteen times greater selectivity for water over methanol and proton conductivity about an order of magnitude lower. (C) 2004 The Electrochemical Society.