A new family of proton-conducting/inorganic hybrid polymer electrolyte membrane was prepared by sol-gel processes. The method involves stabilization of metastable acidic tungsten oxide cluster in the polymer composite membrane. A freestanding, flexible and pale-yellowish transparent membrane was formed from tungsten acidic homogeneous solution by sol-gel condensation with alkoxysilylated polyethylene oxide (PEO). The membrane showed very high protonic conductivity of 10(-2)-10(-3) S cm(-1) from R.T. to 120 degreesC, especially, 1.4 x 10(-2) S cm(-1) at 80 degreesC. The structure of tungsten cluster moieties was neither peroxo complex nor WO3.2H(2)O. It is Suggested that nano-size clusters are dispersed homogeneously in polymer matrix without long-range ordering. Tungsten complexes with carboxylate ligands such as malonic acid could form similar homogeneous membrane. Thermal and ionic transport properties could be controlled by this family of organic derivatives.