The proton-conductive MH2PO4/SiP2O7-based composites (M = K, Rb, Cs) were synthesized, and their structure and proton conductivity were investigated at intermediate temperatures. For all MH2PO4/SiP2O7-based composites, the chemical reaction between MH2PO4 and SiP2O7 has proceeded during the heat treatment at 220 degrees C for 1 h, resulting in the formation of MH5(PO4)(2). The heat-treated composites exhibited higher conductivity than MH2PO4 due to the new conduction phase of MH5(PO4)(2), regardless of the alkaline metal species. The electrolytes of MH5(PO4)(2)/SiP2O7 composites (M=K, Rb, Cs) were also fabricated so as to compare the conductivity with almost the same charge carrier concentration. It is noted that their conductivity is independent of the alkaline metal species. (C) 2007 Elsevier B.V. All rights reserved.