A model for the conduction mechanism in divalent-substituted gamma-BIMEVOXes, of general formula Bi2MxV1 - x O5.5 - (3x/2) -delta (M divalent cation), is proposed. This model is based on the defect structure derived from combined X-ray and neutron powder diffraction studies, which show both tetrahedral and octahedral vanadium polyhedra in the defect vanadate layer. The proposed defect structure can be used to accurately predict solid solution limits in the divalent-substituted BIMEVOXes. The proposed conduction mechanism involves positional exchange of oxide ions and vacancies in the equatorial plane of the vanadate layer and results in the effective two-dimensional movement of the vanadium polyhedra through the structure. Three main features are identified in the structure of the gamma-BIMEVOXes which are fundamental to the exhibition of high oxide ion conductivity shown in these materials. These are, firstly, the polarizibilty and arrangement of the Bi 6s(2) one pairs with respect to vacancies in the vanadate layer; secondly, the well-known variable co-ordination of V in oxide systems; and thirdly, the large vacancy concentration in the vanadate layer.