Electrical conduction of Sr-doped LaP3O9 ([Sr]/{[La]+[Sr]} = 2-10 mol%) was investigated under 0.4-5 kPa of p(H2O) and 0.01-100 kPa of p(O-2) or 0.3-3 kPa of p(H-2) at 573-973 K. Sr-doped LaP3P9 showed apparent H/D isotope effect on conductivity regardless of the Sr-doping level under both H2O/O-2 oxidizing and H-2/H2O reducing conditions at investigated temperatures. Conductivities of the material were almost independent of p(O-2) and p(H2O). These results demonstrated that the Sr-doped LaP3O9 exhibited protonic conduction under wide ranges of p(O-2), p(H2O) and temperature. The conductivity of the Sr-doped LaP3O9 increased with increasing Sr concentration up to its solubility limit, ca. 3 mol%, while the further Sr-doping slightly degraded the conductivity. These indicate that Sr2+ substitution for La3+ leads to proton dissolution into the material and induced protonic conduction. Conductivities of the 3 mol% Sr-doped sample were 2 x 10(-6) - 5 x 10(-4) S cm(-1) at 573-973 K. (c) 2006 Published by Elsevier B.V.