Proton conductive inorganic-organic hybrid membranes were synthesized from 3-glycidoxypropyltrimethoxysilane (GPTMS) and phosphonoacetic acid (PA) with various ratios by a sol-gel process. Self-standing, homogeneous, highly transparent membranes were synthesized. TG-DTA analyses indicated that these membranes were thermally stable up to 200 degrees C. The results of FT-IR and C-13 NMR revealed that phosphonic acid groups of PA were chemically bound to organosiloxane network as a result of reaction between PA and GPTMS. The leach out of phosphonic acid groups from GPTMS-PA to water was reduced compared with phosphoric acid groups from GPTMS-H3PO4. The proton conductivity of the hybrid membranes increased with phosphonic acid content. The conductivity of GPTMS/PA with a 1/1.05 ratio at 130 degrees C was 8.7 x 10(-2) S cm(-1) at 100% relative humidity (RH). (C) 2007 Elsevier Ltd. All rights reserved.