A novel proton-conducting material based on LaP3O9 was developed and its electrical conduction properties were investigated by measuring conductivities under 0.4-5 kPa of p(H2O) and 0.01-100 kPa of p(O-2) at 573-973 K. LaP3O9 conducted protons dominantly by partial substitution of Sr for La. Conductivities of the 1 mol % Sr-doped LaP3O9 were 7 x 10(-7) to 3 x 10(-4) S cm(-1) at 573-973 K, and were relatively higher than those of other proton-conducting rare earth phosphates such as LaPO4 and La7P3O18. Conductivities of the Sr-doped LaP3O9 were almost independent of p(H2O), suggesting dominant protonic conduction even under low p(H2O) conditions. (C) 2004 The Electrochemical Society.