The properties of proton-conducting composite SnP2O7 electrolytes containing LaP3O9 are reported. These electrolytes demonstrate both improved stability and enhanced proton conductivities when compared to their individual constituents. Two different methods were used to prepare samples: sintering or warm-pressing. A conductivity of 1.7x10(-4) S cm(-1) was obtained for warm-pressed SnP2O7-LaP3O9 (Sn:La, 82:18) at 350 degrees C. Under fuel cell conditions the composite with composition Sn:La, 82:18 exhibited the highest open-circuit voltage (OCV) of 0.983 V at 350 degrees C. The OCV values confirmed that the electrolytes were predominantly ionically-conducting.