A study of the kinetics of substitution in [Ru(NH3)(5)H2O](3+) by the nucleophiles [Fe(CN)(6)](4-), [Ru(CN)(6)](4-), [Os(CN)(6)](4-), and [Co(CN)(6)](3-) in the absence of indifferent electrolyte shows that k for substitution (in every case first order in cation concentration) increases as the ratio of the concentration of the nucleophile to the aquo ion increases, and then it abruptly becomes independent of this ratio. When the reaction medium is 1.5 x 10(-2) M H+ (chosen to eliminate catalysis by the formation of [Ru(NH3)(5)H2O](2+) when [Fe(CN)(6)](4-) is the nucleophile), the critical values of R are ca. 4 for [M(CN)(6)](4-), but 8 for [Co(CN)(6)](3-). In a reaction medium adjusted to pH = 3.1, the [M(CN)(6)](4-) species are less fully protonated. Again, approximately constant values of k are reached as R increases, but now at higher values of R. Short of these values of R, maxima in k are observed, which as in the case of earlier studies of oscillator strength of the intervalence absorption bands, are attributed to structural changes, the anions rearranging from positions on the apices of the cation octahedron, to the faces. a study of catalysis of substitution by extrinsic [Ru(NH3)(5)H2O](2+) and, in the case of [Fe(CN)(6)](4-), intrinsic Ru(II) reveals some unusual kinetic effects which also have their origin in ion clustering.