A proton-conducting solid electrolyte membrane based on polyvinyl phosphonic acid (PVPA) and 3-glycidoxypropyltrimethoxysilane was developed via the sol-gel process. The electrical conductivity of the organically-modified membrane obtained with optimum composition was 3.7 x 10(-2) S.cm(-1) at 353 K under 90% of relative humidity (RH), which was sufficient for its practical use as a solid electrolyte for fuel cells. In addition, a membrane based on polyphosphoric acid (PPA) and alkoxysilane was fabricated for comparison by employing polyethylene glycol dimethacrylate (PDM) as an additive. Though the PPA system (with PDM) exhibited similar electrical conductivity to the PVPA system, the latter had a superior mechanical strength and flexibility. Furthermore, the PVPA-based membrane with optimum composition was thermally stable up to ca. 473 K. The membrane exhibited electrical conductivity of 1.3 x 10(-2) S.cm(-1) at 398 K even under 30% of RE, which was similar to that of Nafion (R) at the same temperature under 100% of RH. (C) 2008 Elsevier B.V. All rights reserved.