This study examines the conductivity of the rare earth orthophosphate ceramic series La0.98-xCexSr0.02PO4-delta in oxidizing and reducing atmospheres. The full rare earth orthophosphate series is synthesized and characterized using XRD, SEM, TEM-EDX, and HRTEM to detect the presence of second phases and potential compositional heterogeneities that may affect conductivity. AC impedance measurements under reducing conditions showed that the protonic conductivity is nearly independent of the cerium content. Under oxidizing conditions the conductivity shows percolation-type dependence with increasing Ce content; this is attributed to the appearance of electron-holes, localized on Ce. as the dominant charge carrier. The fit of the experimentally measured conductivity with composition to a percolation model implies a low percolation threshold, which may result from Ce clustering or higher nearest neighbor hopping of the electron holes. The strong Ce concentration dependence of the conductivity suggests an increasing hole concentration with increasing cerium content, which in turn would imply a relatively high stability of the pyrophosphate defects. (C) 2012 Elsevier Ltd. All rights reserved.