The effects of ion bombardment into MgO were investigated by measuring resulting volume changes with a cantilever beam technique and by monitoring the F band absorption induced in the uv region of the spectrum. Single crystals of MgO were bombarded near 〈100〉 with 500-keV argon, which resulted in an expansion of the implanted near-surface layer due to the ion-induced lattice damage. Under subsequent 100-keV proton irradiation, however, a large fraction of this expansion is relieved since the material compacts. This seems to indicate that defects with different charge states are produced in MgO by heavy-ion bombardment and that electronic processes account for the volume changes observed during subsequent irradiation with the primarily ionizing radiation from the 100-keV H+ implantation. Identical behavior was found earlier for the highly ionic Al2O3 while no such effect was observed in the predominantly covalent SiO2. The present results thus corroborate our model of the existence of defects with different charge states in ionic materials. This behavior of MgO and Al2O3 is of considerable interest since both materials are candidates for first-wall application in CTR environments.