Novel proton-conductive inorganic-organic hybrid membranes were synthesized from styryl-substituted alkoxysilanes containing different numbers of methoxy groups and phenylvinylphosphonic acid (PhVPA) through the use of radical copolymerization followed by sol-gel process. The substituted alkoxysilanes were copolymerized with phenylvinylphosphonic acid yielding copolymers that were chemically bound with phosphonic acid groups. The mechanical properties and flexibility of these membranes reflected the type of Si-O-Si linkages, which were controlled through the number of alkoxy groups on the Si atoms in the starting organosiloxane compounds. The proton conductivity of the hybrid membranes increased with increasing phosphonic acid content and with decreasing inorganic Si-O cross-linking. The membrane from difunctional methyl (dimethoxy)silylmethylstyrene/PhVPA = 1/4 exhibited a conductivity of 3.4 x 10(-3) S cm(-1) under about 19.2% R.H. at 130 degrees C. (C) 2007 Elsevier B.V. All rights reserved.