Electrical conduction properties of Sr-doped PrPO4, NdPO4 and SmPO4 with the monazite structure were investigated at 500-925 degrees C under H-2/H2O reducing conditions. From H/D isotope effects and p(H-2)-dependencies of the conductivities, it was found that the materials dominantly conducted protons under the wet reducing conditions although the materials slightly showed n-type electronic conduction at higher temperatures. Based on p(H2O) and p(H-2)-dependencies of the conductivities, defect structures of the materials under the reducing conditions were discussed. As a result, it was considered that major positive defects were oxygen deficits at higher temperatures and protons at lower temperatures. (c) 2006 Elsevier B.V. All rights reserved.