Ytterbium zirconate is of fundamental interest as a model frustrated magnet, and of potential technological importance as a thermal barrier material. In both cases oxygen vacancies are known to play a central role in determining physical properties. We have grown a large single crystal of oxygen- deficient Yb2Zr2O6.5 and we propose to determine its defect structure using diffuse neutron scattering on SXD. The sample adopts the fluorite structure and this opens up the possibility to study magnetic frustration on the FCC lattice. Furthermore the thermal conductivity is known to be anomalously low compared to predictions for random, isolated oxygen vacancies. We shall determine the nature of the oxygen defects in order to assess their role in suppressing the thermal conductivity, and the migration of oxygen at elevated operating temperatures.