X-ray diffraction studies of polycrystalline beryllium oxide irradiated to 1.5 x 1021 nvt (En > 1 Mev) at 110 °C have shown selective symmetric or asymmetric broadening of reflections Me.1 with 1 not zero. Displacements of reflection maxima and centroids occur in such a manner that a unique a a lattice parameter cannot be defined from spacing measurements. Models of clustered interstitial and vacancy defects in a damaged BeO lattice have been proposed and the X-ray scattering from these models has been calculated. Comparison of theoretical and experimental results indicates that the best agreement is obtained for an interstitial cluster model. Some physical implications of this model are discussed.