The conductivity and its ageing behaviour has been determined for zirconia co-doped with 10 mol% of Sc2O3 and 1 mol% CeO2 in different partial pressures of oxygen at 600 degrees C. After 3000 h, samples kept in air, in a humidified mixture of H-2/N-2 and in humidified H-2 exhibited loss in the ionic conductivity of 9%, 19% and 25%, respectively. The conductivity degradation rates after the first 1000 h were 1.5%/1000 h in air and 4.3%/1000 h in humidified H-2. For all atmospheres, after 3000 h at 600 degrees C the conductivity value remained above 10 mS/cm, the minimum value required for viable solid oxide fuel cells. On ageing in reducing conditions, the colour on the surface of the sample changed from white to dark orange, indicating the reduction of Ce4+ cations, which also explains the higher degradation rate in reducing conditions, than in air. The colour gradient within the sample indicates the progress of the reduction front. The core/shell model was applied on the aged reduced sample, and the conductivity of the reduced region is estimated to be 27% lower than the unaged sample. The conductivity behaviour upon re-oxidation is also investigated in air at 600 degrees C. The conductivity recovers significantly in the first 500 h, thereafter increasing linearly with time. After 2000 h of re-oxidation in air, the gain in conductivity is 3.7% compared to the as-aged sample. (C) 2010 Elsevier B.V. All rights reserved.