The water exchange reaction of the polyoxocation GeO(4)Al(12)(OH)(24)(H(2)O)(12)(8+) (K-GeAl(12)) in aqueous solution has been simulated by means of supermolecule density functional calculations. In the proposed dissociative pathway, the leaving water molecule in the first coordination sphere is dissociated from its coordinated aluminium, and via a transition state enters into the second coordination sphere. Our calculated results indicate that the polyoxocation K-GeAl(12) exchanges water in a dissociative way with a barrier height of 37.0 kJ mol(-1), and that the gas-phase model is not suitable to simulate the water exchange reaction of K-GeAl(12) because of the proton-transfer reaction. The calculated results on the transmission coefficients of the water exchange reactions of K-MAl(12) imply that the water exchange reactions of aluminium species with similar structures have close transmission coefficients, but the different charges of aluminium species have an important influence on the transmission coefficients, which may be the reason why K-MAl(12) polyoxocations with different reactivity have nearly the same water exchange rate constants.