Measurements by differential thermal analysis, between room temperature and 1100°C, of the stored energy induced in MgO samples either by irradiation with 1.8 MeV electrons, or plastic deformation or thermal quenching, indicate that these treatments induce a similar type of damage. The energy released ranges between 102oand 1021eV. g−1, which points to the formation of a large number of lattice defects which are aggregated in a variety of clusters or extended defects. Optical absorption measurements of the evolution of anion and cation vacancies after X-irradiation following isochronal annealing up to 1100°C support this proposal.