Hybrid inorganic/organic membranes using a lacunary heteropolyacid proton conductor, H8SiW11O39, in stable polymer matrices based on polyethylene glycol were prepared by a sol-gel method. NMR and IR measurements indicate that this improved sol-gel membrane contains robust covalent bonds between the proton conductor and polymer backbone. However, the polymer in these model systems is not expected to be oxidatively stable. Diffusion coefficients measured for this material, 1.2 x 10(-6) cm(2) s(-1), are of the same order of magnitude as for to that of Nafion(R) whilst ion exchange capacities, 2-2.5 meq g(-1), are twice that of Nafion(R) 117. The temperature dependence of the conductivity increases exponentially with temperature. However, fuel cell performance of this material is poor when compared to a Nafion(R) 117 membrane electrode assembly (MEA). One possible explanation is that there is a lack of organization of the proton conducting elements in the hybrid material. In addition, ex situ membrane conductivity measurements compared to in situ MEA polarization measurements reveal large interfacial resistances in the hybrid membrane MEAs. Further, it appears that these membranes, as currently formulated, require quite stringent humidification requirements for maximum performance. (C) 2004 Elsevier B.V. All rights reserved.