Temperature tolerant proton conducting membranes have attracted much attention recently because of their application to intermediate temperature operation of polymer electrolyte fuel cells (PEFC) with many technological advantages. A new class of amphiphilic organic/inorganic hybrid membranes have been synthesized through sol-get processing of bridged polysilsesquioxanes. Membranes doped with acidic moieties such as 12-phosphotungstic acid (PWA) show large proton conductivities at temperatures up to 160degreesC. In this article, control of the proton conducting properties of the bridged alkylene hybrid membranes have been investigated through modification of sol-gel processes. The conductivity of the hybrid membranes can be shifted by the equivalent PWA weight in the macromolecules and the amount of processing water used for hydrolysis of the monomers. The humidity dependence of the proton conductivity is of great importance, especially for operation above 100degreesC and was found to be dependent on a water activity. A stable conductivity above 100degreesC, which is weakly dependent on the relative humidity, suggests a robust conductive channel structure in the flexible macromolecules. (C) 2003 Elsevier B.V. All rights reserved.