Materials that show fast oxide ion conduction have potential applications as electrolytes in devices such as solid oxide fuel cells (SOFCs), gas sensors and gas separation membranes. While bismuth oxide shows the highest oxide ion conductivity of any material it can only be stabilised at lower temperatures through partial substitution of bismuth by other cations. A total scattering study of W and La substituted bismuth oxide based electrolytes is proposed, with a view to examining details of local coordination and vacancy ordering. Fluorite based phases will be examined in two solid solution systems. Both systems show a highly conducting fully disordered delta-Bi2O3 type phase at elevated temperature, which can be stabilised to room temperature depending on composition. The results from the proposed study will be used to shed light on the conduction mechanisms in these systems.