AbstractNafion, a perfluorosulfonate ionomer, was modified to increase its thermal stability and reduce its methanol permeability. Hybrid membranes of TiO2·SiO2/Nafion and TiO2·SiO2·P2O5/Nafion™ were prepared using an infiltration sol-gel method. Si(OC2H5)4and Ti(OC4H9)4were infiltrated into dry NafionTMmembranes, followed by hydrolysis and condensation reactions in first HCl and then NH4OH solutions. The level of inorganic content was controlled by the infiltration time, incorporating up to 50 wt%. Solvent uptake, swelling, water content and proton conductivity were measured at room temperature. Hybrid membranes of TiO2·SiO2/Nafion with ∼30 wt% of infiltrated oxides showed a significantly lower methanol uptake of ∼20wt% and a swelling ratio of 1.15, as compared to those of unmodified NafionTMmembrane, ∼60wt% for methanol uptake and 1.8 for swelling ratio. Proton conductivities for TiO2·SiO2/Nafion hybrid membranes decreased with increasing infiltrated oxides. However, infiltrated membranes treated in phosphoric acid solutions to increase the number of P-OH groups showed a six-fold increase in proton conductivity.