The ionic current-overpotential characteristics of cathodes on ceria-based electrolytes have been evaluated by galvanostatic current-interruption measurements under fuel cell operating conditions. The effect of either mixed ionic electronic conductivity or high oxygen diffusivity on the oxygen reduction kinetics was investigated. La0.8Sr0.2MnO3 served as a model substance for cathodes with a high activity for oxygen reduction and a low oxygen ionic conductivity. The behaviour of La0.8Sr0.2MnO3 is compared to that of La0.84Sr0.16CoO3 which shows high activity for oxygen reduction and high oxygen diffusivity.