The AC conductivity of eulytite-structured Ba3La(PO4)(3) with 2 mol% Sr substituted for La has been investigated in the range 300-1300 degrees C at different oxygen and water vapor (H2O or D2O) partial pressures. The overall sample impedance has contributions from bulk (grain interior) and grain boundaries. It is concluded from the H/D isotope effect that the bulk conductivity is mainly protonic. The defect chemistry leading to a content of protons is discussed. (C) 2009 Elsevier B.V. All rights reserved.