The structure, energies, and infrared spectra of a range of possible monomeric and dimeric aluminum-containing neutral and anionic species that may be present in either dilute or concentrated alkaline sodium aluminate solutions have been studied using ab initio methods at the MP2/6-31G**, B3LYP/6-31G**, and B3LYP/6-311++G**//B3LYP/6-31G** levels. Results are presented for the species in vacuo and with a representation of the surrounding environment using either a self-consistent polarizable continuum model or a chargecompensating electrostatic potential of varying magnitude to represent high levels of ion concentration. Both predicted energetics and infrared spectra confirm that the dominant species present under dilute aqueous conditions is the Al(OH) 4 (H 2 O) 4 -anion. At high concentrations of sodium aluminate, it is predicted that dimerization to the doubly hydroxyl-bridged species (OH) 3 Al(OH) 2 Al(OH) 3 2-can occur and that this is assisted by the coordination of two or more water molecules.