Samples of the BICDVOX system, formulated as Bi4CdxV2-xO11-(3x/2)-delta in the Cd substitution range 0 <= x <= 0.25 were synthesized using the standard solid state reaction. The correlation between phase stability and oxide ion performance were investigated by variable temperature XRPD, DSC and AC impedance spectroscopy. The substitution of V5+ by Cd2+ exhibited different phase transitions upon varying composition. For compositions with x <= 0.05, two successive transitions; alpha <-> beta <-> gamma are evident, while the beta <-> gamma transition exists in the composition range 0.05 < x < 0.175. However, some temperature dependent phenomena confirmed the exixtence of the gamma' <-> gamma transition, coupled with the tetragonal symmetry stabilization for x >= 0.175. The maximum oxide ion conductivity at lower temperatures was observed for x = 0.20. It has also been found that the slow V4+ -> V5+ re-oxidation results in increased defect trapping effects in the system at higher temperatures. (c) 2012 Elsevier B.V. All rights reserved.