There has been recent interest in the use of cermets based on stabilised delta bismuth oxide based solid electrolytes as cathodes in solid oxide fuel cells. It has been shown that double substitution into Bi2O3 can result in delta-phase stabilisation, where similar levels of doping by a single dopant cannot. This has been explained in terms of an increase in configurational entropy in the di-substituted systems. Our recent studies on the system Bi2O3-PbO-Y2O3 have confirmed that delta-phase stabilisation does occur when double substitution is carried out . The resulting defect cubic fluorite phases possess theoretical vacancy concentrations higher than that in delta-Bi2O3. The aim of the present proposal is to investigate subtle phase transitions in the Bi2O3-PbO-Y2O3 system using RMC modelling of total neutron scattering data.