Electrical conduction in La7P3O18 was investigated by using conductivity measurements under 0.4-5 kPa of p(H2O), and 0.01-100 kPa of p(O-2) at 500-930 degreesC. The conductivity of La7P3O18 was considerably enhanced by Sr substitution for La. The Sr-doped La7P3O18 had higher conductivities in wet than dry conditions, and exhibited the apparent H/D isotope effect on conductivity. From these results, it was found that La7P3O18 conducted protons dominantly by substituting Sr for La and that conducting protons were introduced through equilibrium between the material and the water vapor in ambient atmosphere. Results of conductivity measurements under various p(H2O) and p(O-2) suggested that p-type electronic conduction became significant in addition to protonic conduction with increasing temperature, increasing p(O-2) and decreasing p(H2O). The conductivity behavior of Sr-doped La7P3O18 was rather similar to that of acceptor-doped LaPO4, although the conductivity of Sr-doped La7P3O18 was lower than that of acceptor-doped LaPO4. (C) 2004 Elsevier B.V. All rights reserved.