The proton-conductive electrolytes of CsH5(PO4)2/SiP2O7 composites were synthesized, and composite effects of silicon pyrophosphate as a supporting matrix at intermediate temperatures were investigated by comparing the properties of CsH5(PO4)(2)/SiO2 composite. Although both composites showed similar thermal stability, the temperature dependence of the conductivity was quite different each other; the conductivity of the composite electrolyte of CsH5(PO4)(2)/SiP2O7 was about one-order magnitude higher at every temperature investigated and the maximum conductivity achieved was 116 mS cm(-1) at 230 degrees C. These results suggested that the interfacial interaction between the proton-conductor phase of CsH5(PO4)(2) and the matrix of SiP2O7 played an important role in the proton conduction mechanism. (c) 2005 Elsevier Ltd. All rights reserved.