Constrained Car-Parrinello molecular dynamics simulations and thermodynamic integration have been performed for an associative pathway of water exchange between aqueous [UO2(OH2)(5)](2+) and bulk water. The simulated free energy of activation for this process, 6.7 kcal mol(-1), is significantly lower than that computed for a purely dissociative mechanism, 10.8 kcal mol(-1). Because the transient hexahydrate is indicated to have no chemically significant lifetime, the exchange mechanism can be classified as associative interchange.