Electrical conduction properties of Sr-doped LaPO4 and Sr-doped CePO4 under H2O/O2 and H2O/H 2 conditions were investigated with conductivity measurements. Conductivities of 1 mol % Sr-doped LaPO4 were 10.2 - 103.5 S cm-1 at 500-925°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 102.8-102.0 S cm-1 at 500-925°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 Ce 4+ caused by substituting Sr2+ for Ce3+. Under wet reducing conditions, however, conductivities of the material decreased to 10-5.2-10-3.43.4 S cm-1 at 500-925°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.