In search for better ionically conducting ceramics for oxygen separators, fuel cells and sensors, the electrical conductivity and microstructure of the 9 mol% (Sc2O3-Y2O3)-ZrO2 system with varying Sc2O3/Y2O3 ratios has been investigated in detail with XRD, SEM, TEM and conductivity measurements as a function of temperature. The stability of electrolyte compositions was studied by continuously monitoring conductivity as a function of time at 850 and 1000 degrees C. Impedance spectroscopy was employed for determining the contribution of the grain boundary resistivity. The role of alumina additions to selected Sc2O3-Y2O3-ZrO2 compositions was studied as alumina is known to reduce the grain boundary resistivity by scavenging silica impurities and enhance mechanical properties in zirconia-based systems. Al2O3-containing compositions show much higher conductivity degradation compared with alumina-free materials. This behaviour has been investigated in more detail with XRD and TEM analysis.