An investigation of electrical conductivity and defect structure in the fluorite related system Bi3+xNb0.8W0.2O7.1+3x2 (0.0 <= x <= 2.0) using a.c. impedance spectroscopy and powder neutron diffraction is presented. All compositions exhibit incommensurate ordering of the cubic fluorite sublattice, with only the x = 2.0 composition showing evidence for an order-disorder transition above 650 degrees C. Non-linear behaviour in the thermal expansion of the cubic lattice parameter and the Arrhenius plot of total conductivity are discussed in terms of the defect structure. (C) 2010 Elsevier B.V. All rights reserved.