The mechanism for the water-exchange reaction with the transition metal aqua ions from Sc-III through Zn-II has been investigated. The exchange mechanisms were analyzed on the previously reported model (Rotzinger, F. P. J. Am. Chem. Soc. 1996, 118, 6760) that involves the metal ion with six or seven water molecules. The structures of the reactants/products, transition states, and penta- or heptacoordinated intermediates have been computed with Hartree-Fock or CAS-SCF methods. Each type of mechanism, associative, concerted or dissociative, proceeds via a characteristic transition state. The calculated activation energies agree with the experimental Delta(G) double dagger(298) or Delta H double dagger(298) values, and the computed structural changes indicate whether an expansion or compression takes place during the transformation of the reactant into the transition state. These changes are in perfect agreement with the changes deduced from the experimental volumes of activation (Delta double dagger(298)) The motions of the ligands involved in the exchange reaction are described by the imaginary vibrational mode. All these computed quantities allow the attribution of the water-exchange reactions to the A, I-a, or D mechanisms with use of the terminology of Merbach (Merbach, A. E. Pure Appl. Chem. 1982, 54, 1479). Within the present model, no transition state has been found for the I-d mechanism. It remains to be verified, using an improved model, whether it really does not exist. The dissociative mechanism is always feasible, but it is the only possible pathway for high-spin d(8), d(9), and d(10) systems. In contrast, the associative mechanism requires that the transition metal ion does not have more than seven 3d electrons. Thus, Sc-III, Ti-III, and V-III react the A, Ni-II, Cu-II, and Zn-II via the D (or I-d) mechanism, whereas for the elements in the middle of the periodic table, the high-spin 3d(3)-3d(7) systems, both associative (I-a/A) and dissociative (D) pathways are feasible. The present results suggest that for Sc-III hexa- and heptacoordinated species could coexist in aqueous solution.