Electrical conduction in 1 mol% Sr-doped (La0.99Ce0.01)PO4 was investigated at 500-925 degrees C under H-2/H2O and H2O/O-2, conditions. Conductivities of the material at the investigated temperatures were 10-(5.1) -10(-3.6) S(.)cm(-1) under the wet reducing condition, and 10(-5.4) -10(-3.7) S(.)cm(-1) under the wet oxidizing condition. H/D isotope effect on conductivity confirmed dominant protonic conduction under the both conditions. Defect structures in the material were discussed from partial pressure dependency of the conductivity. Consequently, it was suggested that Ce4+ C, at Ce3+ sites mainly compensated acceptor charge under oxidizing conditions, although the material primarily conducted protons. On the other hand, under reducing condition, protons and oxygen deficits became dominant positive defects while reducing Ce3+ to Ce4+ (c) 2005 Elsevier B.V. All rights reserved.