Composite solid electrolytes (1-x)CSH5(PO4)(2)/xSiO(2) (x=0-0.75) based on silica with different specific surface areas have been studied by the complex impedance, DSC, and X-ray diffraction methods. The proton conductivity of composites is shown to depend on the silica's specific surface area: the higher the specific surface area the stronger influence on the transport properties of composites. The maximal composite conductivity was 2.3 mS.cm(-1) at T = 140 degrees C, which exceeded the conductivity of pure CsH5(PO4)(2) by more than one order of magnitude. The drastic changes in the transport properties of composites are governed by the salt dispersion and/or amorphisation, which are confirmed by the changes in thermodynamic parameters. An H-2/air fuel cell based on CSH5(PO4)(2)/SiO2 demonstrates a maximum power density of 8 mW/cm(2) at U=0.3 V and a current density up to 80 mA/cm(2) at a short circuit. (c) 2008 Elsevier B.V. All rights reserved.