A systematic study of the conductivity of polyvinylidene fluoride (PVDF) and CsHSO4 composites, containing 0 to 100% CsHSO4, has been carried out. The polymer, with its good mechanical properties. served as a supporting matrix for the high proton conductivity inorganic phase. The conductivity of composites exhibited a sharp increase with temperature at 142 degrees C, characteristic of the superprotonic phase transition of CsHSO4. At high temperature (160 degrees C), the dependence of conductivity on vol % CsHSO4 was monotonic and revealed a percolation threshold of similar to 10 vol %. Al low temperature (100 degrees C), a maximum in the conductivity at similar to 80 vol % CsHSO4 was observed. Results of preliminary fuel cell measurements are presented.