Ageing effects in the defect fluorite structure of Bi3Nb1-xYxO7-x have been studied using powderneutron diffraction. Two compositions at x=0.4 and x=0.6 were studied after annealing for prolonged times (400-500h) at 410 and 550 degrees C. The results show that significant changes in oxide ion distribution occur at 550 degrees C, but that at 410 degrees C only the lower x-value composition exhibited such a change. A comparison of lattice parameter variation with temperature for annealed and unannealed samples suggests that at higher temperatures the oxide ion distribution is independent of thermal history. (c) 2007 Elsevier B.V. All rights reserved.