Bismuth oxide doped with isovalent rare earth cations retains the high temperature defective fluorite structure upon cooling down to room temperature, However, these doped materials undergo an order-disorder transition of the oxygen sublattice at about 600 degreesC. When annealed at temperatures less than the transition temperature, the oxygen sublattice continues to order, and consequently oxygen ion conductivity undergoes a decay, However, the conductivity activation energies of the ordered structures after extended aging at 500 degreesC were observed to be lower than those of the structures prior to aging. Modeling of ordered structures based on TEM diffraction patterns indicates a < 111 > vacancy ordering in the anion sublattice, Neutron diffraction studies show additional structural changes in the oxygen sublattice due to ordering. These studies indicate that the ionic conductivity is dependent on the distribution of oxygen ions between the regular 8c sites and the interstitial 32f sites in the fluorite structure, Based on the TEM and neutron diffraction studies and conductivity activation energies of the ordered and disordered structures, a transport mechanism for oxygen ions through interstitial positions is proposed,