Electrical conduction properties of Sr-doped LaPO4 and Sr-doped CePO4 under H2O/O-2 and H2O/H-2 conditions were investigated with conductivity measurements. Conductivities of 1 mol % Sr-doped LaPO4 were 10(-5.2)-10(-3.5) S cm(-1) at 500-925 degrees C under wet reducing conditions and were close to those under wet oxidizing conditions. It was found from the H/D isotope effect on conductivity that the material showed dominant protonic conduction under wet reducing conditions. As for 1 mol % Sr-doped CePO4, conductivities were 10(-2.8)-10(-2.0) S cm(-1) at 500-925 degrees C under wet oxidizing conditions and much higher than those of 1 mol % Sr-doped LaPO4 under the same conditions. Such high conductivities of Sr-doped CePO4 seemed attributable to electronic conduction due to partial oxidation of Ce3+ to Ce4+ caused by substituting Sr2+ for Ce3+. Under wet reducing conditions, however, conductivities of the material decreased to 10(-5.2)-10(-3.4) S cm(-1) at 500-925 degrees C, which was almost comparable with those of 1 mol % Sr-doped LaPO4. It was concluded that Sr-doped CePO4 showed mixed protonic and p-type electronic conductions under wet reducing conditions. Based on the results of conductivity measurements, defect structures in Sr-doped LaPO4 and CePO4 under wet oxidizing and reducing conditions are discussed. (c) 2005 The Electrochemical Society. All rights reserved.