We have determined the defect structure of oxygen-depleted Y2Ti2O7 using diffuse neutron scattering, and we find that these oxygen vacancies are also the dominant defects in as-grown, nominally stoichiometric samples. Annealing in oxygen eliminates the diffuse scattering. We predict that these oxygen vacancies are also the dominant defects in spin-ice Ho2Ti2O7. Thus we propose to study the defect structures of oxygen-deficient, as-grown and annealed Ho2Ti2O7 by neutron diffraction and diffuse neutron scattering using SXD. Measurements will be performed at 30K in order to minimise thermal diffuse scattering. The results will be compared with first-principles DFT calculations using CASTEP. The importance of these results is that these structural defects are associated with magnetic defects that are capable of explaining the anomalous magnetic monopole dynamics at sub-Kelvin temperatures.