Praseodymium doped cerium oxide (PrxCe1 - xO2 - delta : PCO) exhibits mixed ionic and electronic conductivity at relatively high oxygen partial pressures (e.g. air) and enhanced oxygen storage capacity resulting from the ready reduction/oxidation of tri- and tetra-valent Pr. A change in the redox state of Pr results in a color change of PCO, and so serves as a means of investigating the Pr oxidation state, oxygen non-stoichiometry, and position of the Pr energy level within the ceria band gap. The wavelength dependent optical absorptivity was examined as a function of the Pr content (x) and oxygen partial pressure. Based on an analysis of the optical absorption spectra, the Pr level was found to lie approximately 1.6 eV below the ceria conduction band for x = 0.1 and 0.2, consistent with a defect model derived from electrical and thermogravimetric measurements on PCO. (c) 2012 Elsevier B.V. All rights reserved.