Pr-doped ceria (PCO) is a mixed ionic-electronic conductor (MIEC) under oxidizing conditions, with excellent ionic and relatively low electronic conductivity. Motivated by investigations of model systems, PCO is examined with regards to the performance as an SOFC air electrode. The conductivity of Pr0.2Ce0.8O2.delta (20PCO) in air is examined in detail, and a novel technique based on high-temperature X-ray diffraction (HT-XRD) to identify the exact temperature at which half of all Pr-dopants are ionized is presented. The redox-state of the Pr dopants strongly influences the polaron mobility, which can be linked to an increase of the polarization resistance of symmetric cells with 20PCO electrodes, indicating a change of the rate-limiting step for the oxygen reduction reaction (ORR).