Cobalt oxide films are of technological interest as magnetic substrates that may support the direct growth of graphene, for use in various spintronic applications. In this work, we demonstrate the controlled growth of both Co 3 O 4 (111) and CoO (111) on Ru (0001) substrates. The growth is performed by Co molecular beam epitaxy, at a temperature of 500 K and in an O 2 partial pressure of 10− 4 Torr for Co 3 O 4 (111), and 7.5× 10− 7 Torr for CoO (111). The films are distinguished by their dissimilar Co 2p x-ray photoemission (XPS) spectra, while XPS-derived O/Co stoichiometric ratios are 1.33 for Co 3 O 4 (111) and 1.1 for CoO (111). Electron energy loss (EELS) spectra for Co 3 O 4 (111) indicate interband transitions at∼ 2.1 and 3.0 eV, while only a single interband transition near 2.0 eV is observed for CoO (111). Low energy electron diffraction (LEED) data for Co 3 O 4 (111) indicate twinning during growth, in contrast to the LEED data for CoO (111). For Co 3 O 4 (111) films of less than 20 Å average thickness, however, XPS, LEED and EELS data are similar to those of CoO (111). XPS data indicate that both Co oxide phases are hydroxylated at all thicknesses. The two phases are moreover found to be thermally stable to at least 900 K in UHV, while ex situ atomic force microscopy measurements of Co 3 O 4 (111)/Ru (0001) indicate an average surface roughness below 1 nm. Electrical measurements indicate that Co 3 O 4 (111)/Ru (0001) films exhibit dielectric breakdown at threshold voltages of∼ 1 MV cm− 1. Collectively, these data show that the growth procedures yield Co 3 O 4 (111) films with topographical and electrical characteristics that are suitable for a variety of advanced device applications.