Water-exchange reactions of the hexahydrated 3d ions have been studied using large basis set SCF computations on isolated penta- (in trigonal bipyramidal or square pyramidal conformation), hexa-, and heptahydrated metal ion complexes, the latter with one single M-O distance and two angular coordinates optimized. A dissociative mechanism has been modeled by using the dissociation energy of the sixth water ligand in a gas-phase process as the activation energy in an Arrhenius plot versus the logarithm of the experimental water-exchange rate in solution, under the assumption that the energy contributions from solvent effects are similar within a series of ions. For the divalent 3d ions (V2+ to Zn2+), the remarkably good linear correlation found, in combination with the fairly constant solvation energies estimated in the preceding article, strongly supports an essentially dissociative mechanism in solution. This is not compatible with the associative interchange mechanisms proposed for Mn2+ and V2+ on the basis of their negative experimental activation volumes, Delta V double dagger, for which an alternative explanation is proposed. For the trivalent ions Ti3+, V3+, Cr3+, Fe3+, and Ga3+, the four first also with negative Delta V double dagger values, a corresponding plot gives a poor correlation. An associative mechanism has been investigated by using the binding energy of the seventh water ligand in a similar Arrhenius plot. The divalent 3d ions showed no correlation, supporting the previous conclusion of a dissociative mechanism. The trivalent 3d ions with negative hr values gave an acceptable correlation with an associative interchange model, while the Ga3+ ion previously reported to react dissociatively, deviated as expected. Comparisons of calculated and experimental activation enthalpies supported an intermediate water interchange mechanism with increasingly associative character in the order Cr3+, Fe3+, V3+, and Ti3+.