The water exchange mechanisms on the hexaaqua ions of Al3+, Ga3+, and In3+ in aqueous solution have been modeled by using ab initio calculations at the Hartree-Fock level. As an approximation aqua clusters in vacuo involving seven water molecules were considered. For species with five, six, or seven water molecules in the first coordination shell of the cation, stable intermediates and transition states have been optimized and characterized from vibrational analyses. Water exchange reaction pathways could then be proposed via interconnected intermediates and transition states. The calculations provide theoretical evidence for a break in kinetic behavior between Al3+ Ga3+ on one side and In3+ on the other. Hexaaqua complexes of Al3+ and Ga3+ show no tendency to increase their coordination number over six arid, despite the high positive charge on the central ion, water exchange proceeds via a D mechanism involving a pentacoordinated intermediate [M(OH2)(5).(OH2)](3+). This is in agreement with experimental volumes of activation Delta V double dagger that are the highest measured up to date for trivalent metal cations. For [In(OH2)(6)](3+) a dissociative exchange reaction is in principle feasible, but an associative A mechanism via a 7-fold coordinated reactive intermediate [In(OH2)(7)](3+) is energetically much more favorable. Theoretical arguments and indirect experimental evidence in favor of an A and against an I, mechanism are discussed. The experimentally still lacking activation volume Delta V double dagger for water exchange on [In(OH2)(6)](3+) has been predicted to be -5 +/- 1 cm(3)mol(-1). The computed activation energies Delta E double dagger for Al3+ and Ga3+ are in remarkable quantitative agreement with the experimental values for Delta H double dagger. This lends support to the applied theoretical model and suggests that for this class of aqua ions, with a spherical and strongly bound first hydration shell, cluster calculations in vacuo are a viable approach to reproduce the structural changes and the activation parameters for water exchange reactions in aqueous solution. Attempts have been made to determine the water exchange rate at In-(aq)(3+), Lu-(aq)(3+), and Zn-(aq)(2+) by using Tb-(aq)(3+) as a shift reagent. While none of these attempts proved successful, new lower limits for water exchange on these three ions can be given as 1 x 10(7) (In3+), 1 x 10(7) (Lu3+), and 5 x 10(7) s(-1) (Zn2+).