Organic inorganic composite proton-conducting membranes were prepared by using a hybrid organic inorganic matrix Compoceran (R) E201 that consists of an epoxy prepolymer and a siloxane part. It was demonstrated that the compound disperses proton-conducting Cs-substituted phosphotungstic acid (Cs-PWA) particles uniformly in spite of the large difference in densities. Relatively high proton conductivity of 10(-3) S cm(-1) was obtained at 160 degrees C and at RH of 80% for the membrane with the weight ratio Cs-PWA/E201 = 1.6.