Fluorite-type fcc phases have been synthesised in the system Bi2O3-Er2O3-PbO by solid state reaction, and a partial air-quenchable domain of the fluorite-type phase has been established. Some of these materials display high oxide ion conductivities, notably (BiO1.5)(0.80)(ErO1.5)(0.11)(PbO)(0 09) and (BiO1.5)(0.85)(ErO1.5)(0.12)(PbO)(0.03), which have conductivities of 0.49 and 0.72 S cm(-1) at 750 degrees C, respectively, placing them among the most conductive Bi2O3-based materials. Conductivity was found to increase with increasing Pb2+/Er3+ ratio and decreasing (Er3+ + Pb2+)/Bi3+ ratio. Positional disorder in the oxide ion sublattice was characterised by neutron powder diffraction. At room temperature, the oxide ion sublattice appeared to be completely disordered, with oxide ions only in 32f and 48i sites, and changes in occupancy with increasing Pb2+/Er3+ and (Er3+ + Pb2+)/Bi3+ ratios were not significant. At 700 degrees C, there appeared to be oxide ions in 8c sites for the material (BiO1.5)(0.80)(ErO1.5)(0.11)(PbO)(0 09), with a correspondingly smaller occupancy of the 32f sites, whilst the occupancy of the 48i sites had not changed significantly. (c) 2007 Elsevier B.V. All rights reserved.