The ionic conductivity of Sn0.9In0.1P2O7 ceramic was investigated under various atmospheres within the temperature range of 130-230 degrees C. Similar to mixed-conductive perovskite oxides at high temperatures (such as SrCe0.95Yb0.05O3-alpha,La0.9Sr0.1Ga0.8Mg0.2O3-alpha at 600-1000 degrees C), Sn0.9In0.1P2O7 can conduct both protons and oxide ions at low temperatures (130-230 degrees C). The conductivity of Sn0.9In0.1P2O7 reaches 0.019 S/cm at 200 degrees C in wet nitrogen. Its transport numbers determined by steam concentration cells are around 0.76 for a proton and 0.12 for an oxide ion. The performance of direct methanol fuel cells at 170 degrees C using mixed-ion conductive Sn0.9In0.1P2O7 electrolyte is higher than that at 235 degrees C using pure proton conductive CsH2PO4 electrolyte. This is attributed to direct oxidation of CO at the anode by the oxide ions generated at the cathode and moved through the Sn0.9In0.1P2O7 electrolyte. (c) 2008 The Electrochemical Society. [DOI: 10.1149/1.2988135] All rights reserved.