The water-exchange rate constants and activation parameters for the [M(CH3NH2)5H2O]3+ (M = Cr(III), Co(III), Rh(III)) complexes, determined by variable-temperature and -pressure O-17 NMR are respectively as follows : k(ex)298 = (4.1 +/- 0.5) x 10(-6), (700 +/- 80) x 10(-6), and (10.6 +/- 0.6) x 10(-6) s-1; DELTAH(double dagger) = 98.5 +/- 3, 99.0 +/- 6, and 112.7 +/- 2 kJ mol-1; DELTAS(double dagger) = -17.5 +/- 10, +26.7 +/- 22, and +37.8 +/- 6 J K-1 mol-1; DELTAV(double dagger) = -3.8 +/- 0.3, +5.7 +/- 0.2, and +1.2 +/- 1.1 cm3 mol-1. These results indicate a clear differentiation in the intimate substitution mechanism operating for these complexes. For the Cr(III) complex, a clearly associative activation mode operates, and for the Co(III) analog, a clearly dissociative activation mode operates, while the borderline nature of the Rh(III) complex is quantified by an activation volume value practically zero. The differences in the values obtained for DELTAV(double dagger) and for k(ex)298 as compared with those corresponding to the analogous [M(NH3)5H2O]3+ Complexes are interpreted in view of a shift to more dissociatively (or less associatively) activated mechanisms operating for the complexes with larger amine groups. That is, the increase in the steric congestion around the metal center causes the mechanism to be shifted to the dissociatively activated side of the I(a) <-> I(d) mechanistic continuum.