We prepared Ce1-xPrxO2-delta and investigated its high temperature properties as a material for the composite active layer of a solid oxide fuel cell (SOK) cathode. We found that increasing the Pr concentration increases the total conductivity and oxygen vacancy concentration at high temperature, and this may lead to improvement of the cathodic reaction. When Ce1-xPrxO2-delta is heated, it expands significantly at a certain temperature (T-inf), and this expansion depends on the Ce1-xPrxO2-delta composition. The expansion is associated with an abrupt increase in the unit cell volume of the cubic structure. Abrupt increases in the total conductivity and oxygen vacancy concentration were also observed at T-inf. These results can be explained by oxygen vacancy and electron formation (n-type electronic conductivity increase) at T-inf and above T-inf. A sample with a Ce1-xPrxO2-delta composition where x = 1.0 has no such T-inf. A composition near x = 1.0 for Ce1-xPrxO2-delta is favorable for the active layer material of an SOK cathode, because of the high conductivity and high concentration of oxygen vacancies. (C) 2011 Elsevier B.V. All rights reserved.