Various alkali metal pentahydrogen diphosphates, MH5(PO4)(2) (M=K, Rb, Cs), were mixed with several molar ratios, and the mixed alkali effect on thermal and electrochemical properties was investigated at intermediate temperatures. The XRD patterns of mixed MH5(PO4)(2) indicated the formation of substitutional solid solution. The mixing of three kinds of alkali metal phosphates with equimolar ratio, KH5(PO4)(2)-RbH5(PO4)(2)-CsH5(PO4)(2), was the most effective for the depression of melting point. The mixed MH5(PO4)(2) and their composites with SiP2O7 matrix attained higher conductivity in the low temperature range (80-150 degrees C) due to the lower melting point of alkali metal phosphates as compared to the corresponding CsH5(PO4)(2) samples. The ionic transference numbers of mixed MH5(PO4)(2)/SiP2O7 composites achieved 0.88-0.92, indicating that these materials were proton conductors. (C) 2012 Elsevier B.V. All rights reserved.