Proton exchange membrane fuel cells (PEMFCs) have been considered to be a key technology for renewable energy systems such as an alternative battery for electric vehicles (EVs), stationary power generators, and mobile phones. Intermediate temperature operation of PEMFCs overcome most of the technological problems of the current PEM system such as CO poisoning, cathode overpotential, and complicated water and heat management. As a membrane is recognized to be a key element for more efficient PEMFCs, new classes of polymer electrolytes have been investigated elsewhere. In this paper, a new class of amphiphilic organic/inorganic hybrid membranes have been synthesized through sol-gel processing of bridged polysilsesquioxanes. The membrane doped with acidic moieties such as 12-phosphotungstic acid shows large protonic conductivities at intermediate temperatures up to 140degrees C and was found to be flexible as well as thermally stable due to the temperature tolerant inorganic frameworks in the hybrids. The fast ionic transport through the hybrid macromolecules has been possibly ascribed to nanosized phase separation of amphiphilic macromolecules. The proton conductivity of the hybrid was 3 x 10(-2) S/ cm at 140degrees C without degradation, which can potentially be used as an advanced PEM electrolyte membrane for intermediate temperature operation.