For the low-spin t(2g)(6) Ru(OH2)(6)(2+) (DeltaV(double dagger) = -0.4 cm(3) mol(-1)) and Rh(OH2)(6)(3+) (Vi(double dagger) = -4.2 cm(3) mol(-1)) hexaaquaions, the respective I-d and I-a water exchange mechanisms had been assigned, mainly on the basis of activation volumes DeltaV(double dagger) and entering ligands effects for water substitution. For Ru(II) the near-zero DeltaV(double dagger) was supposed to be due to the compensation between a positive contribution (the loss of a water molecule) and a negative one (the contraction of the bonds of the five spectator ligands at the transition state). Recently, it has been suggested that Rh(III), because of its higher positive charge, could promote further spectator ligands bond contraction sufficient to change the sign of DeltaV(double dagger) to a negative value. If true, this would be an example of Limitation in the use of DeltaV(double dagger) for a direct diagnosis of the mechanism. Quantum chemical calculations including hydration effects show that the activation energies for the water exchange on Rh(OH2)(6)(3+) via the I-a (114.8 kJ mol(-1)) and the D pathways is 21.8 kJ mol(-1) in favor of the former. In the case of Ru(OH2)(6)(2+) all attemps to compute a transition state for an interchange mechanism failed, but the calculated DeltaE(double dagger) for the D mechanism (71.9 kJ mol(-1)) is close to both experimental DeltaG(298)(double dagger) and DeltaH(298)(double dagger) values. The calculated Delta Sigmad(M-O) values of -0.53 Angstrom for rhodium(III) and +1.25 Angstrom for ruthenium(II) agree with the experimented DeltaV(double dagger) values and suggest I-a and D (or I-d) mechanisms, respectively. In the case of Ru(OH2)(6)(2+) the shortening of the bonds of the five spectator ligands to reach the transition states corresponds to a volume change of -1.7 cm(3) mol(-1). For Rh(OH2)(6)(3+) these spectator ligands' volume decrease is much smaller (maximum of -0.8 cm3 mol(-1)) and the bond lengths of the two exchanging ligands at the transition state are characteristic of an interchange pathway with a small "a" character. Because of the strong Rh-III-O bonds, water exchange on Rh(OH2)(6)(3+) proceeds via the I-a pathway with retention of the configuration, whereas the same reaction of Ru(OH2)(6)(2+), which has considerably weaker Ru-II-O bonds, follows the Icl or the D mechanism.