a b s t r a c tThe growth and characterization of epitaxial Co 3 O 4 ð1 1 1Þ films grown by oxygen plasma-assisted molecular beam epitaxy on single crystal a-Al 2 O 3 ð0 0 0 1Þ is reported. The Co 3 O 4 ð1 1 1Þ grows single crystalline with the epitaxial relation Co 3 O 4 ð1 1 1Þ½1 21ka-Al 2 O 3 ð0 0 0 1Þ½1010, as determined from in situ electron diffraction. Film stoichiometry is confirmed by X-ray photoelectron spectroscopy, while ex situ X-ray diffraction measurements show that the Co 3 O 4 films are fully relaxed. Post-growth annealing induces significant modifications in the film morphology, including a sharper Co 3 O 4 =a-Al 2 O 3 interface and improved surface crystallinity, as shown by X-ray reflectometry, atomic force microscopy and electron diffraction measurements. Despite being polar, the surface of both as-grown and annealed Co 3 O 4 ð1 1 1Þ films are ð1 Â 1Þ, which can be explained in terms of a surface inversion in the spinel structure.