A study of electrical and structural characteristics of compositions in the Bi3Ta1-xNbxO7 system, using X-ray and neutron powder diffraction and AC impedance spectroscopy, is presented. The electrical conductivity increases with increasing niobium content. A full solid solution is observed which adopts an incommensurately ordered pseudo-cubic fluorite structure (type II). Analysis of the defect structure of the x=0.50 composition shows chains of niobate/tantalate octahedra as a likely structural motif. A small degree of non-linearity in the thermal expansion of the cubic subcell lattice parameter is observed. (C) 2012 Elsevier B.V. All rights reserved.