We report a comparative study of transport and thermodynamic properties of CSH2PO4 and (1-x)CsH2PO4/XSiO2 composite electrolytes (x=0.2-0.7) based on various types of silica. The role of acidic centers of silica surface for chemical and thermal stability Of CSH2PO4 in composites at 200-230 degrees C has been shown. Silica modified by small amounts of CsHSO4 and H3PO4 has been obtained. Heterogeneous doping by modified silica causes a significant increase of the low temperature conductivity of CsH2PO4 due to disordered state of salt. The composite systems with modified silica exhibit high proton conductivity, 10(-3)10(-2) S/cm at 130-250 degrees C and have higher thermal stability at lower water vapor partial pressures comparing with the pure salt. Contrary to the former, composites based on highly dispersed SiO2 (S-sp similar to 41-520 m(2)/g) with pH similar to 7 exhibit the low proton conductivity at T > 220 degrees C due to strong interface interaction and CS2H2P2O7 formation. The phase composition of the systems which depends on x has been determined. The investigations have been carried out by means of X-ray diffraction, differential scanning calorimetry, IR and complex impedance spectroscopy at different atmosphere humidity. (c) 2007 Elsevier B.V. All rights reserved.