Density-functional theory (DFT) methods were used for investigations on the aluminum-hexaaquo complex and on aqueous aluminum-acetate complexes. Solvent effects were computed by means of the polarized continuum model (PCM). Extensive basis set studies and comparison of several functionals lead to efficient and accurate procedures which were applied to the computation of aluminum-acetate complexes. A variety of structural arrangements such as monodentate or bidentate with respect to the bonding of the carboxylate group or cis/trans with respect to the relative position of two acetate molecules were considered. ΔH and ΔG values were calculated for the substitution reaction of water molecules in the aluminum-hexaaquo complex by acetate anions. Characteristic differences in ΔS were found depending on the number of water molecules released per acetate. Overall, we find that monodentate structures are only slightly preferred over bidentate ones and that we expect a relatively complicated system of chemical equilibria without any dominant complex.