The molecular dynamics of the fluorite-phase of bismuth sesquioxide has been investigated using previously-developed interionic potentials. The effect of system size has been examined by using systems containing 270, 640, 1250 and 2160 particles which correspond to a system of n × n × n unit cells of Bi4O6with n = 3, 4, 5 or 6. Phase changes to a superlattice in which there are 3n/2 oxygen planes along one of the cubic axes occurred with n = 4 or 6 but not with n = 5. For n = 3 a phase change to a different structure occurred. However, the diffusion coefficient of D determined from the velocity autocorrelation function does not depend strongly on superlattice formation.