The effect of yttria co-doping of scandia zirconia on stabilisation and ionic conductivity has been investigated. Compositions in the ternary system (Y2O3)(x)(Sc2O3)((11-x))(ZrO2)(89) (x = 0-11) were prepared by solid state reaction and characterised by XRD, SEM and impedance spectroscopy. The electrical conductivity was studied as a function of temperature. The stability of the electrolyte materials was examined at the intermediate solid oxide fuel cell (SOFC) temperature of 800 degreesC for up to 1500 h. The contribution of the bulk and grain boundary resistivity of sintered and long term annealed compositions to total resistivity was estimated. Yttria additions were found to improve the phase stability of scandia stabilised zirconia. Even 1 mol% Y2O3 addition eliminates the rhombohedral phase (Sc2Zr7O17, the beta-phase) and stabilises the cubic structure at room temperature. The best overall ionic conductivity was observed for compositions containing 2 mol% Y2O3. The 1 and 2 mol% Y2O3 Compositions exhibit high electrical conductivities and good stabilities. Typically two linear re,ions were observed in Arthenius conductivity plots. The lower temperature region exhibited an activation energy of around 1.3 eV and the high temperature region 0.75 eV. The difference between these activation energies decreases as yttria content increases and thus we attribute this change to short range ordering effect rather than simple near neighbour associations. (C) 2004 Elsevier B.V. All rights reserved.