There is currently considerable research interest in the synthesis and characterisation of materials with high oxide-ion conductivities, owing to their potential exploitation as solid electrolytes in Solid Oxide Fuel Cells (SOFCs) for future power generation applications. Attention has recently focussed on the ZrO2-Sc2O3 system, which has one of the highest oxide-ion conductivities in the temperature range ~800-900K. This proposal will investigate the defect structure of selected ZrO2-Sc2O3 compositions, including studies of the degradation in the conductivity with ageing and a comparison of the disorder with the current ¿best¿ material (ZrO2-Y2O3). This structural information will be correlated with studies of the ionic conductivity measured by a.c. impedance measurements and computer simulations of the ionic diffusion processes.