Double substitution of bismuth in bismuth oxide is investigated in the system Bi3Nb1-xErxO7-x using X-ray and neutron diffraction and a.c. impedance spectroscopy. A full solid solution is observed with the basic fluorite structure maintained throughout. Long-range superlattice ordering observed at low x-value compositions is replaced by short-range ordering as the erbium content increases. Nb5+ and Er3+ dopant cations are believed to adopt a distorted octahedral coordination geometry, with evidence for clustering of these ions. Significant compositional changes are observed in the oxide ion distribution, but there is little significant thermal change in the average oxide ion distribution to account for observed non-linear behaviour in the thermal expansion of lattice parameter and Arrhenius plot of total conductivity. These phenomena may well be associated with more subtle changes in local ordering of vacancies. (C) 2010 Elsevier B.V. All rights reserved.