Abstract : Pd nanophases in a Nafion polymer membrane electrolyte were used to enhance DMFC performance by preventing or reducing methanol crossover through the electrolyte. Interestingly, the Pd-impregnated nanocomposite membrane showed a lower permeability for methanol and maintained a proton conductivity comparable to that of a pure Nafion membrane. Because of the modification of the membrane by the Pd nanophases, a high molar concentration of methanol could be applied for use in practical DMFC without any power loss.