Conductivity of several compositions in the scandia-zirconia system (Sc2O3 content between 7.0 and 11.0 mol%) has been investigated as a function of temperature and time at the nominal solid oxide fuel cell operating temperature of 850 degreesC. For comparison, the stability of electrolyte compositions was also studied at 1000 degreesC by monitoring conductivity as a function of time. All compositions showed conductivity degradation as a function of time both at 850 and 1000 degreesC, however, it was much lower in compositions containing Sc2O3 between 9.0 and 11.0 mol% especially at 1000 degreesC. The maximum conductivity was observed for materials with compositions close to 9 mol% Sc2O3-ZrO2. The 10.0 and 11.0 mol% Sc2O3-ZrO2 compositions showed the presence of a rhombohedral-phase (Sc2Zr7O17, the beta-phase) at room temperature. Hysteresis loops were observed in the conductivity-temperature plots around 500-600 degreesC due to the transformation of the low conducting beta-phase to high conductivity cubic-phase on heating and the reverse on cooling. The contribution of grain boundary impedance in as-sintered and annealed specimens was relatively small for all specimens studied.