The structures of the transition states and intermediates formed in the water-exchange of hexaaqua complexes of the first row transition elements have been computed with ab initio methods at the Hartree-Fock or CAS-SCF level. As an approximation, water molecules in the second coordination sphere except one, bulk water, and anions have been neglected. For each of the three types of activation, namely associative, concerted, and dissociative mechanism, a representative transition metal complex has been studied, viz. Ti(OH2)(6)(3+), V(OH2)(6)(2+), and Ni(OH2)(6)(2+). Each type of mechanism proceeds via a characteristic transition state.