The long-term conductivity stability is tested on zirconia based electrolyte materials for solid oxide fuel cell applications. The ageing studies have been performed on the samples of ZrO2 co-doped with 10 mol% of Sc2O3 and 1 mol% MO2, where M = Ce or Hf (denoted respectively 1Ce10ScSZ and 1Hf10ScSZ) in oxidising and reducing atmospheres, at 600 degrees C for 3000 h. At 600 degrees C, these compositions show initial conductivity of around 9-12 mS-cm(-1) in air. After 3000 h of ageing. no phase transitions are observed in any of the samples. For the first 1000 h, the degradation rate is higher than in the subsequent 2000 h; thereafter, conductivity degrades linearly with time for all samples. In air, the loss in the conductivity is lower than in reducing conditions. The 1Ce10ScSZ shows the highest degradation rate of 3.8%/1000 h in wet H-2/N-2 after the first 1000 h of ageing. A colour change of the 1Ce10ScSZ sample from white to dark orange after ageing in reducing conditions indicates the reduction of Ce4+ cations. The high degradation rate in the 1Ce10ScSZ sample aged in reducing conditions is explained by the formation of Ce3+ cations. The colour gradient is observed within the sample suggestive of the slow reduction process in 1Ce10ScSZ. The inhomogeneous 1Ce10ScSZ sample is represented using a core-shell model, where the core is the unreduced portion inside the reduced shell. The model allows the conductivity of the reduced and aged shell to be determined from the total conductivity; this is also measured on a thin sample subjected to the similar ageing treatment. The conductivity behaviour upon reoxidation of the samples aged in reducing conditions is also investigated in air at 600 degrees C for 2000 h. The observation of conductivity recovery in 1Ce10ScSZ supports our view that it is due to Ce re-oxidation. The Ce concentration across the grain boundary is also studied using transmission electron microscopy. No clear evidence of segregation was observed in 1Ce10ScSZ, possibly due to the low Ce content of the sample. (C) 2011 Elsevier B.V. All rights reserved.