Praseodymium doped ceria, PrxCe1-xO2 - delta (PCO), exhibits many interesting and unusual properties including a pO(2) dependent ionic conductivity, an anomalously large thermal expansion coefficient in air, and a significant electronic conductivity component at elevated pO(2). These unusual features are discussed in terms of the variable valent nature of Pr at elevated pO(2), its position within the ceria band gap and the creation of an impurity band supporting small polaron transport. Implications for use of PCO as a ceria solid electrolyte compatible cathode, stress induced chemical expansion and optically detected redox kinetics are analyzed and discussed. (c) 2012 Elsevier B.V. All rights reserved.