MgO was deposited on different planes of sapphire substrates by Molecular Beam Epitaxy (MBE). Cross-sectional specimens from these systems were prepared and investigated by high resolution electron microscopy (HREM). Two different orientation relationships were obtained for MgO films deposited on the differently oriented sapphire substrate. Misfit dislocations at the interface in each case were analyzed. MgA12O4 (spinel) forms at the MgO/A12O3 interface by a solid state reaction under electron beam irradiation.