A new proton-conductive electrolyte consisting of x NH4PO3 / TiP2O7 (x = 0.2- 1.0) was synthesized for use in intermediate-temperature fuel cells, and its electrochemical properties were studied in the temperature range 150-300 degreesC. For each composite electrolyte, the X-ray diffraction pattern was identical to that for TiP2O7, and NH4PO3 was not detected due to a change to an amorphous state at sintering temperature. The proton conductivity of the composite electrolyte was improved by increasing the molar ratio of NH4PO3 under both dry and wet atmospheres. These results indicate that NH4PO3 is responsible for high proton conductivity and that TiP2O7 serves as a supporting matrix. However, under each atmosphere, the temperature dependence of proton conductivity showed non-Arrhenius-type behavior and was almost reversible. Higher proton conductivities were observed at 200-250 degreesC and 150-200 degreesC under dry and wet atmospheres, respectively. For x = 1.0, the highest proton conductivity was determined to be 24.2 mS cm(-1) at 250 degreesC and 66.0 mS cm(-1) at 200 degreesC under dry and wet atmospheres, respectively. Fuel cells that used 1.0 NH4PO3 / TiP2O7 (electrolyte thickness 2.5 mm) as an electrolyte were fabricated and showed good performance at 250 degreesC. The maximum power density was 24.4 mW cm(-2) when dry hydrogen and dry oxygen were used as the fuel and oxidant, respectively. (C) 2004 The Electrochemical Society.