The behavior of surface undercoordinated Al3+ cations as electron acceptors is investigated by means of periodic ab initio calculations. The (0001) surfaces of alpha-Al2O3 and AlN(w) were chosen as representative of the different Al coordination types. The electronic structure was analyzed through the valence localized Wannier functions that allow a straightforward description of the electron pairs in terms of chemical concepts. The relevance of the electron back-donation toward surface Al3+ cations in the reconstruction of the surfaces is discussed, and evidence was found that some pi-bond character is displayed by the surface Al-X bonds after relaxation. This explains the tendency to form surface Al-X planar layers in this kind of system.