The influence of electrochemical pretreatment on the long-term behaviour of electrodes of the type 'oxygen, platinum/stabilized zirconia' was investigated. Working electrodes comprising a massive platinum contact on yttria-stabilized zirconia were equilibrated in an oxidizing gas phase and examined by means of impedance spectroscopy in the temperature range from 773 to 1173 K and in the oxygen partial pressure range from 1 to 101 325 Pa. After polarizing the working electrodes under applied voltages of - 2.0 or + 0.5 V against the reference electrode at 873 K in air and admitting relaxation periods of at least 250 h, the impedance analysis was repeated in the temperature interval between 773 and 873 K and in the same oxygen partial pressure interval as before. A significant increase of the electrode conductance was observed in both cases. The results indicate that the electrochemical performance of zirconia-based electrodes is not only determined by the catalytic properties of the metal, but in addition by the catalytic properties of the electrolyte.