Proton conducting polymer composite membranes were prepared by casting method from borosiloxane electrolyte containing sulfonic acid groups and partially sulfonated poly(ether-sulfone) (SPES). Proton conductivity of the composite membranes exhibited high ionic conductivity, stability to oxidation, and good film forming ability. Silica coating on the composite membrane improved water resistance. (C) 2004 Elsevier Ltd. All rights reserved.