Electronic stability in aluminum clusters is typically associated with either closed electronic shells of delocalized electrons or a +3 oxidation state of aluminum. To investigate whether there are alternative routes toward electronic stability in aluminum oxide clusters, we used theoretical methods to examine the geometric and electronic structure of AlnOm (2 <= n <= 7; 1 <= m <= 10) clusters. Electronically stable clusters with large HOMO-LUMO (highest occupied molecular orbital and lowest unoccupied molecular orbital) gaps were identified and could be grouped into two categories. (1) Al(2)nO(3)n clusters with a +3 oxidation state on the aluminum and (2) planar clusters including Al4O4, Al5O3, Al6O5, and Al6O6. The structures of the planar clusters have external Al atoms bound to a single O atom. Their electronic stability is explained by the multiple-valence Al sites, with the internal Al atoms having an oxidation state of +3, whereas the external Al atoms have an oxidation state of +1.