Zirconia-ceria ceramics are promising materials within automotive exhaust catalysts, exploiting the Ce3+Ce4+ redox reactions to store oxygen. For a Zr:Ce ratio of 1:1 it is possible to prepare materials with a long-range ordered array of Zr4+ and Ce4+ ('filled pyrochlore structure', kappa-CeZrO4), or with a disordered arrangement of the cations in a tetragonal distortion of the fluorite structure (t'-Zr0.5Ce0.5O2). Both can be reduced fully to to Ce3+ and, remarkably, the cation sublattice remains ordered/disordered in each case as oxygen is removed. However, kappa-CeZrO4 shows far superior oxygen storage properties, with O2- migration occuring more rapidly and at lower temperatures than in t'-Zr0.5Ce0.5O2. A neutron diffraction study of both systems as a function of oxygen partial pressure at high temperature on Polaris will attempt to explain this observation.