Density functional theory calculations have been performed for the water exchange mechanism of aquated Al(III). The effect of pH was considered by studying the exchange processes for [Al(H2O)(6)](3+) and its conjugated base, [Al(H2O)(5)OH](2+). Both complexes were found to exchange water in a dissociative way with activation energies (E-A) of 15.9 and 10.2 kcal/mol, respectively. The influence of solvent molecules on the gas-phase cluster model was considered by the addition of up to four water molecules to the model system. The stabilizing effect of the solvent on the transition state decreases E-A to 8.6 (hexa-aqua complex) and 7.6 (monohydroxo complex) kcal/mol, whereas E-A for all hydroxo species is consistently significantly lower than those for the related aqua systems, which indicates a much faster water exchange rate. For the hydroxo complex, all calculated five-coordinate intermediates, nH(2)O center dot[Al(H2O)(4)(OH)](2+) (n = 1, 2, 3, 4, 5), are more stable than the corresponding six-coordinate reactants. Our results therefore suggest the presence of a stable five-coordinate species of aquated Al(III), namely, the [Al(H2O)(4)(OH)](2+) complex.