By controlled oxidation of a clean NiAl(110) surface an epitaxial thin layer of Al2O3 may be grown. The oxide layer shows a sharp low-energy electron diffraction pattern of two domains of nearly rectangular unit cells. A stable tunneling current was obtained for a wide range of sample bias voltages U (from a few mV up to nearly 10 V). For U in the mV range the images essentially contain contributions of the interface layer, which can be measured with atomic resolution. For large U (e.g., 4 V) the electronic states of the oxide film contributed additionally to the substrate electrons, which allows an accurate analysis of the growth mode of the oxide film.