Novel proton-conductive inorganic-organic hybrid membranes were synthesized from (trimethoxysilyl methyl) styrene (TMSMS) and phenylvinylphosphonic acid (PhVPA) mixed at various ratios using radical copolymerization followed by the sol-gel process. Self-standing, homogeneous, highly transparent membranes were synthesized. TG-DTA analysis indicated that these membranes were thermally stable up to 180 degrees C. FT-IR and C-13 NMR revealed that the functionalized alkoxysilane was copolymerized with phenylvinylphosphonic acid. The phosphoric acid groups of the TMSMS/PhVPA copolymer were chemically bound to the organosiloxane network formed via the hydrolysis and polycondensation reaction between the silanol groups of TMSMS. The proton conductivity of the hybrid membranes increased with the phosphorric acid content. The TMSMS/PhVPA membrane of a 1/4 ratio revealed conductivities of 3.7 x 10(-2) S cm(-1) under 100% R.H. at 130 degrees C and 9.5 x 10(-4) S cm(-1) under about 19.2% R.H. at 130 degrees C. (c) 2007 Elsevier B.V. All rights reserved.