The water exchange on the ruthenium(III) aqua pentaammine ion was investigated using quantum chemical techniques. Calculations were performed for the free and the hydrated ions, with and without electron correlation (dispersion was neglected). The most accurate result was obtained when both hydration and correlation were considered. Two interchange pathways were investigated, one proceeding with retention of the configuration and the other one giving rise to stereomobility. The latter is unlikely to operate, because of its high activation energy. The computed energy of activation for the exchange with retention of the configuration (88 kJ/mol) is close to the measured Delta H-298(double dagger)/Delta G(298)(double dagger) values (91.5 and 93.8 kJ/mol, respectively). The dissociative mechanism does not operate because of its higher activation energy (103 kJ/mol).