Accurate nuclear density distribution of cubic bismuth oxide (delta-Bi2O3) has been obtained by the maximum entropy method (MEM)-based pattern fitting combined with the Rietveld method using neutron-powder-diffraction data. The result shows for the first time that the oxide ions have a complicated disorder spreading over a wide area and shift to the <111> directions from the ideal fluorite site. These features are responsible for the high oxide-ion conductivity. (C) 2003 Elsevier B.V. All rights reserved.