Nanosized proton-conducting Sn0.95In0.05P2O7 ionomers were grown on carbon supports by the coprecipitation of tin and indium chlorides with excess ammonia water and subsequent solid-state reaction with phosphoric acid. The resulting homogeneous networks of Sn0.95In0.05P2O7 ionomers enhanced the activity of a Pt catalyst for the oxygen reduction reaction in the temperature range of 150-250 degrees C. In addition, the Pt catalyst showed high corrosion resistance in the cathode environment while maintaining high performance levels. A comparison of the cathode performance between Sn0.95In0.05P2O7 and H3PO4 ionomers further demonstrated that the Sn0.95In0.05P2O7 nanoparticles are promising candidates for cathode ionomer materials at high temperatures.