Composite electrolytes of NH4PO3/pyrophosphate (NH4PO3/ZrP2O7, NH4PO3/Sr2P2O7, and NH4PO3/TiP2O7) with various molar ratios were fabricated, and their thermal and electrochemical properties were compared at intermediate temperatures. The XRD pattern of NH4PO3/Sr2P2O7 composite was consistent with a mixed phase of crystalline NH4PO3 and Sr2P2O7 regardless of the composition ratio, whereas those of the other composites were identical to pyrophosphates. A significant difference in conductivity was observed depending on the supporting matrices of pyrophosphates although each composite contained almost the same molar concentration of NR4PO3. Among the composites, NH4PO3/ZrP2O7 (molar ratio; 1:1) exhibited the highest proton conductivity, which was more than twice that of NH4PO3/TiP2O7 (1:1). The conductivity of NH4PO3/Sr2P2O7 (2:1) composite was 2-3 orders of magnitude lower than that of NH4PO3/ZrP2O7 (1:1). These results suggest that the surface property of pyrophosphates strongly affects the electrochemical properties of composites. Furthermore, a fuel cell that used NH4PO3/ZrP2O7 composite as an electrolyte was successfully demonstrated at 300 degrees C. (C) 2007 Published by Elsevier B.V.