Recent research has shown that bismuth oxide based electrolytes are viable components in solid oxide fuel cells. Di-substitution of Bi3+ by an isovalent and a subvalent cation leads to stabilisation of the highly conducting fluorite delta-phase of Bi2O3. This leads to a system with nominally higher vacancy concentration than Bi2O3 itself, with higher stability than the mono-substituted systems. The present proposal seeks to investigate the oxide ion distribution in the Bi2O3-PbO-Er2O3 system and its thermal variation, with a view to explaining its conductivity behaviour. While the long range structure will be examined by conventional Rietveld analysis, a total scattering study will be carried out on the Yb substituted analogue to yield information on local structure and vacancy ordering.