The long-term ionic conductivity behaviour of samples of zirconia co-doped with 10 mol.% of Sc2O3 and 1 mol.% CeO2 is evaluated in oxidizing and reducing atmospheres at 600 A degrees C. After 3,000 h, the sample kept in reducing atmospheres exhibits 20% loss in the ionic conductivity, while the sample kept in air shows 6% degradation. No phase transitions were observed in the samples after both the ageing studies. The main contribution towards the loss in the ionic conductivity of the sample kept in air comes from grain boundaries; however, for the sample aged in reducing conditions, both grain and grain boundary contribute similarly towards the increase in the total resistivity. This is tentatively explained by the reduction of Ce4+ cations, dissolved in the fluorite lattice of ZrO2.