Density functional theory calculations using split-valence polarized basis sets augmented by diffuse s and p functions on the carbon and oxygen atoms were carried out for the complexes of hydrated Al3+ with oxalic acid, and the oxalate mono- and dianions. Monodentate and bidentate structures with up to three ligands have been computed. The polarized continuum model was used to study the solvent effect on the structures and stabilities of the complexes. Reaction energies for the replacement of water molecules in the aluminum-hexaaquo complex by oxalate ligands have been computed. Based on a detailed thermodynamical analysis, characteristic differences in the formation of mono- and bidentate structures were found. In the former case the entropy contributions to ΔG are rather small, whereas in the latter case they are substantial. Thus, the formation reactions for the monodentate complexes are energy-driven whereas those for the bidentate complexes are entropy-driven. In agreement with experiment, the most dominant complex in solution is [AlOx3]3-. From the complexes with oxalic acid and the partially deprotonated form HOx-, only the latter should give stable bidentate complexes in solution.