Novel fast proton-conducting organic-inorganic nanocomposite membranes were successfully fabricated. The polymer matrix obtained through proper oxidation of thiol ligands in (3-mercaptopropyl)trimethoxysilane (MPTS) and hydrolysis/condensation reaction of (3-glycidoxypropyl)trimethoxysilane (GPTS) showed relatively high proton conductivity over 10(-2) S/cm at 25 degrees C. The proton conductivities of the fabricated composite membranes increased up to 3.6 x 10(-1) S/cm by increasing temperature and relative humidity to 70 degrees C and 100% relative humidity. The high proton conductivity of the composites is due to the proton conducting path through the GPTS-derived pseudo-polyethylene oxide network in which sulfonic acid ligand work as proton donor. (C) 2005 The Electrochemical Society.