Stabilised bismuth oxides are of interest as intermediate temperature solid electrolytes for a variety of electrochemical device applications. Lead doping into bismuth yttrate yields defect fluorite structures that have vacancy concentrations that exceed that in the parent d-Bi2O3. The aims of this proposal are to determine the temperature and compositional dependence of defect structure in the systems Bi4-xPbxYO7.5-x/2 and Bi5-xPbxYO9-x/2, to examine the long range and short range ordering of defects in these systems using a total scattering approach and to correlate these results with the electrical conductivity in order to yield information on the conduction mechanism in these systems.