To prepare a crosslinked hybrid polymer electrolyte membrane (PEM) with high chemical stability, a silane monomer, namely p-styryltrimethoxysilane (StSi), was first grafted to poly(ethylene-co-tetrafluoroethylene) (ETFE) film by gamma-ray preirradiation. Hydrolysis-condensation and sulfonation were then performed on the StSi-g-rafted ETFE (StSi-g-ETFE) films to give them crosslinks and proton conductibility, respectively. Thus, a crosslinked proton-conducting hybrid PEM was obtained. The crosslinks introduced by the silane-condensation have an inorganic -Si-O-Si-structure, which enhance the chemical and thermal stabilities of the PEM. The effect of the timing of the hydrolysis-condensation (before or after sulfonation) and the sulfonation method (by chlorosulfonic acid or H2SO4) on the properties of the resulting hybrid PEMs such as ion-exchange capacity, proton conductivity, water uptake, chemical stability, and methanol permeability were investigated to confirm their applicability in fuel cells. We conclude that the properties of the new crosslinked hybrid StSi-grafted PEMs are superior to divinylbenzene (DVB)-crosslinked styrene-grafted membranes. (C) 2008 Wiley Periodicals, Inc.