The frustrated magnetism of pyrochlores makes them candidates for applications in magnetic cooling and scientific instrumentation, and the proposal that they can be used to manipulate isolated magnetic monopoles has generated huge scientific and technological interest. The roles of structural defects are key to understanding the low-temperature properties of these materials. Defective pyrochlores are also of interest for fuel cells, catalysts for lithium-oxygen batteries, and their resistance to radiation damage makes them attractive materials for radioactive waste storage. The phonon dispersions from this class of material display anomalous features which are attributed to the presence of defects. We now plan to study a large, highly defective crystal using inelastic neutron scattering in order to determine the effect of oxygen vacancies on the lattice dynamics.