The microstructure of magnesium aluminate spinel (MgAl²O⁴) and alumina (Al²O³) has been examined with transmission electron microscopy following ion irradiation to damage levels of 0.1 to 5 keV/atom (1 to 50 dpa) at room temperature and 650°C. The ion irradiation produced interstitial dislocation loops of types α/4(110}{110} and α/4{110){111} in spinel along with a very low density of α/6(111){111} loops. Dislocation loops of types α/3[0001] (0001) and α/3[1100]{1100} were tentatively identified in alumina. The loop size increased and the density decreased gradually with increasing fluence in spinel irradiated at 650°C, with the net result that the concentration of interstitials contained in the loops remained nearly constant at ∼0.1 at%. Defect-free regions were observed adjacent to grain boundaries, and the irradiated surface in spinel irradiated at 650°C. The denuded zone width was very small for spinel irradiated at 25°C and Al²O³ irradiated at 650°C. For a given irradiation temperature, the loops in spinel were larger and of much lower density than the loops in Al²O³.