The structure of Bi3TaO7 has been examined by X-ray and neutron powder diffraction. The structure is that of an ordered defect fluorite and has been refined using a cubic delta-Bi2O3 type subcell model. A defect structure model is proposed with Ta5+ in predominantly a distorted octahedral geometry. These tantalate octahedra are thought to form corner sharing chains in a similar way to that proposed in the isostructural phase Bi3NbO7. The thermal variation of conductivity and lattice parameter appear to be correlated with subtle changes in the oxide ion distribution. (c) 2008 Elsevier B.V. All rights reserved.