Electrical conduction in the monazite-type LaPO4 doped with 1 mol% of alkaline earth metals (Mg, Ca, Sr, and Ba) was investigated by using conductivity measurements under 0.4-5 kPa of p(H2O) and 0.01 - 100 kPa of p(O-2) at 500-925 degreesC. The Sr-doped LaPO4 exhibited highest conductivities among the four samples. Conductivities of the Ca-doped LaPO4 were in same levels as those of the Sr-doped sample, but the Ba-doped LaPO4 showed conductivities in almost half magnitude compared with the Sr- and Ca-doped samples. Conductivities of the Mg-doped LaPO4 were much lower than those of the other doped samples, and were comparable with those of the undoped LaPO4. From humidity dependence and H/D isotope effect of the conductivity, it was found that the Ca-, Sr-, and Ba-doped LaPO4 conducted protons dominantly under moisturized conditions, whereas the Mg-doped LaPO4 did not show significant protonic conduction. The differences in conduction properties depending on the doped alkaline earth metals could be basically interpreted by the differences in the solubility of the alkaline earth metals into LaPO4. The P(H2O) and p(O-2) dependencies of the conductivities of the Ca-, Sr-, and Ba-doped LaPO4 were finally discussed in terms of defect equilibria. (C) 2004 Elsevier B.V. All rights reserved.