A new proton-conductive composite of NH4PO3-(NH4)(2)Mn(PO3)(4) was synthesized and characterized as a potential electrolyte for intermediate temperature fuel cells that operated around 250 degrees C. Thermal gravimetric analysis and X-ray diffraction investigation showed that (NH4)(2)Mn(PO3)(4) was stable as a supporting matrix for NH4PO3. The composite conductivity, measured using impedance spectroscopy, improved with increasing the molar ratio of NH4PO3 in both dry and wet atmospheres. A conductivity of 7 mS cm(-1) was obtained at 250 degrees C in wet hydrogen. Electromotive forces measured by hydrogen concentration cells showed that the composite was nearly a pure protonic conductor with hydrogen partial pressure in the range of 10(2)-10(5) Pa. The proton transference number was determined to be 0.95 at 250 degrees C for 2NH(4)PO(3)-(NH4)(2)Mn(PO3)(4) electrolyte. Fuel cells using 2NH(4)PO(3)-(NH4)(2)Mn(PO3)(4) as an electrolyte and the Pt-C catalyst as an electrode were fabricated. Maximum power density of 16.8 mW/cm(2) was achieved at 250 degrees C with dry hydrogen and dry oxygen as the fuel and oxidant, respectively. However, the NH4PO3-(NH4)(2)Mn(PO3)(4) electrolyte is not compatible with the Pt-C catalyst, indicating that it is critical to develop new electrode materials for the intermediate temperature fuel cells. (c) 2006 Elsevier Ltd. All rights reserved.