We have investigated the crystallographic and electronic structure of gamma-alumina surfaces obtained by thermal oxidation of Al foil. By combining X-ray photoelectron spectroscopy, electron energy loss spectroscopy, transmission electron microscopy and transmission electron diffraction investigations, we have shown that such gamma-alumina surfaces are mainly (100), (1 $($) over bar$$ 10) and (112) oriented and are characterized by a specific electronic structure in the band gap. The appearance of defect levels decreases the gap from 8.7 eV to 2.5 eV at the surface. These features are correlated to the ionicity of the gamma-alumina surface and could explain their chemical activity (i.e. acido-basic properties).