The stability regime of the oxide ion conductor delta-Bi2O3 is limited to a narrow temperature range just below its melting point. Substitution of Bi3+ by a variety of isovalent and aliovalent cations can lead to stabilisation of fluorite type phases to room temperature. The structures of these phases show both commensurate and incommensurate ordering of the fluorite lattice as well as distortion in some cases. We propose a detailed investigation of defect structure in bismuth rich compositions in the systems Bi14WxLa1-xO24-3x/2 and Bi14TaxY1-xO23.5-x. In particular we seek to explain observed curvature in the Arrhinius plot of conductivity and the appearance of in intermediate phase in these di-substituted systems.