Epitaxial cubic MgO thin films have been deposited on single crystal Si(001) substrates by metalorganic molecular beam epitaxy. The Mg source was the solid precursor magnesium acetylacetonate and a rf excited oxygen plasma was the oxidant. The growth process involved initial formation of an epitaxial beta-SiC interlayer followed by direct deposition of a MgO overlayer. The films were characterized by in situ reflection high energy electron diffraction, x-ray diffraction, conventional and high resolution transmission electron microscopy, atomic force microscopy, Auger electron spectroscopy, and Fourier transform infrared spectroscopy. The beta-SiC interlayer had an epitaxial relationship such that SiC(001)parallel to Si(001) and SiC [110]parallel to Si [110]. The SiC interlayer showed a columnar grain structure with planar defects including twin bands and stacking faults. The MgO overlayer showed an epitaxial relationship given by MgO(001)parallel to Si(001) and MgO[110]parallel to Si[110]. No evidence of twins in the MgO layers was observed.