Tungsten doping into Bi7Nb2O15.5 was investigated using a combination of X-ray and neutron diffraction and ac impedance spectroscopy. Only limited solid solution formation is observed in the system Bi7Nb2-2xW2xO15.5+x, with a defect fluorite type phase observed up to around x=0.2. The defect structure of this phase shows, that Nb5+ and W6+ ions are likely to adopt a distorted octahedral coordination geometry, with Bi3+ ions in predominantly four-pyramidal geometry. Non-linearity in the Arrhenius plot of conductivity as well as the thermal variation of the cubic lattice parameter are interpreted in terms of a subtle redistribution of oxide ions in the structure at elevated temperatures. (c) 2007 Elsevier B.V. All rights reserved.