SnP2O7-based proton conductors were characterized by Fourier transform infrared spectroscopy (FTIR), temperature-programmed desorption (TPD), X-ray diffraction (XRD), and electrochemical techniques. Undoped SnP2O7 showed overall conductivities greater than 10(-2) S cm(-1) in the temperature range of 75-300 degrees C. The proton transport numbers of this material at 250 C under various conditions were estimated, based on the ratio of the electromotive force of the galvanic cells to the theoretical values, to be 0.97-0.99 in humidified H-2 and 0.89-0.92 under fuel cell conditions. Partial substitution of In3+ for Sn4+ led to an increase in the proton conductivity (from 5.56 x 10(-2) to 1.95 x 10(-1) S cm(-1) at 250 degrees C, for example). FTIR and TPD measurements revealed that the effects of doping on the proton conductivity could be attributed to an increase in the proton concentration in the bulk Sn1-xInxP2O7. The deficiency of P2O7 ions in the Sn1-xInxP2O7 bulk decreased the proton conductivity by several orders of magnitude, which was explained as due to a decrease in the proton mobility rather than the proton concentration. The mechanism of proton incorporation and conduction is examined and discussed in detail. (c) 2006 The Electrochemical Society.