A proton-conductive electrolyte based on a CsH2PO4/SiP2O7 composite was synthesized, and its electrochemical properties were investigated in the temperature range of 110-287 degrees C. Addition of SiP2O7 as supporting matrix brought about the structural change of conduction phase, CsH2PO4, into CsH5(PO4)(2). The resultant composite electrolyte showed high thermal stability and conductivity under 30% H2O/Ar atmosphere. The maximum conductivity achieved was 44 mS cm(-1) at 266 degrees C. Furthermore, the temperature dependence of the conductivity showed no conductivity-jump inherent in the CsH2PO4-based composite. (C) 2005 The Electrochemical Society.