Ageing effects in the defect fluorite structure of Bi 3 Nb 1−x Y x O 7−x have been studied using powder neutron diffraction. Two compositions at x=0.4 and x=0.6 were studied after annealing for prolonged times (400–500h) at 410 and 550°C. The results show that significant changes in oxide ion distribution occur at 550°C, but that at 410°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.