A detailed study of the temperature and compositional variation of defect structure in doped bismuth tantalates is proposed. Tantalum doping into Bi2O3 can achieve d-phase stabilisation at various Bi2O3:Ta2O5 ratios, yielding fluorite based compounds with high conductivity. By increasing the Bi2O3:Ta2O5 ratio and further doping with Y3+ an increase in vacancy concentration can be achieved, whilst maintaining the basic fluorite structure. The temperature dependence of defect structure will be compared with observed non-Arrhenius behaviour in conductivity.