An investigation of electrical conductivity and defect structure in the fluorite related system Bi 3+x Nb 0.8 W 0.2 O 7.1+3x/2 (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°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.