Conductivity measurements of mixed cesium hydrogen disulphate phosphate were carried out. Additional thermodynamic data on pure Cs-3(HSO4)2H(2)PO(4) were obtained. New highly conductive (50-200 degrees C) composite proton electrolytes based Oil Cs-3(HSO4)(2)H2PO4 and silica were synthesized. The properties of (1-x)CS3(HSO4)(2)H2PO4/xSiO(2) composites (x =0-0.95) were studied by differential scanning calorimetry, X-ray powder diffraction and complex impedance methods. Heterogeneous doping was shown to increase the low temperature phase conductivity Of Cs-3(HSO4)(2)H2PO4 by more than one order of magnitude. The observed jump in conductivity becomes diffuse, shifts to lower temperatures and disappears at x=0.7. The conductivity of Cs-3(HSO4)(2)H2PO4/SiO2 composites depends on composition, reaching a maximum at similar to 15-35 vol.% silica and then decreasing due to a percolation effect. A disordered state Of Cs-3(HSO4)(2)H2PO4 With increased proton conductivity was realized for highly dispersed silica. The conductivity of the Cs-3(HSO4)(2)H2PO4/SiO2 composite was shown to exceed that of the CsHSO4/SiO2 systems by one order of magnitude. (c) 2005 Elsevier B.V. All rights reserved.