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Microstructures of ion irradiated magnesium-aluminate spinel (MgAl₂O₄) and rutile (TiO₂) were examined in detail using cross-sectional transmission electron microscopy (TEM). Single crystalline (111)-oriented MgAl₂O₄ and (100)-oriented rutile wafers were used for irradiation experiments. Based on selected area electron diffraction patterns, high-resolution TEM images, and their Fourier transforms, the following results were obtained. (1) Prior to amorphization, an ordered MgAl₂O₄ spinel transforms into a disordered rock salt-like phase, in which all cations reside on octahedral interstices within a pseudo-cubic close-packed oxygen anion sublattice. (2) Ion beam irradiation of rutile induces a reoriented surface layer which consists of rutile crystallites, some with the same orientation as the original substrate, and some with different orientations compared to the original single crystal.