The activation parameters for the aquation of Co(NH3)(5)Cl2+ and Cr(NH3)(5)Cl2+ are almost equal, and the strongly negative volumes of activation, Delta V-double dagger (-9.9 and -10.6 cm(3) mol(-1) for Co and Cr, respectively), might at the first glance suggest a common associative mechanism for both complexes. Computations of the corresponding reaction coordinates indicate that, in agreement with Swaddle and co-workers' studies and as expected, a dissociative interchange mechanism (I-d) operates for cobalt(III), but an associative interchange one (I-a) for chromium(III). For these two asymmetric reactions, the bond lengths changes delta(Co) and delta(Cr) that are involved in the formation of the transition state and defined with respect to the fully concerted I pathway allow the attribution of the reaction mechanism. The signs of delta(Co) and delta(Cr), being positive and negative, suggest a d and a activation for the above aquations.