The ligand substitution reactions of co-ordinated H2O in aquacobalamin (vitamin B12) by OCN–, SCN–, SeCN–, NO2–, S2O32– and N3– in a NaNO3 medium, in contrast to an earlier report on some of these ligands in a KCl medium, proceeded too rapidly for saturation kinetics to be observed. The discrepancy, as suggested previously, is due to formation of a significant fraction of the more substitution-inert chloro complex in a KCl medium. Second-order rate constants, corrected for pH effects, for the reactions have been determined as a function of temperature, from which activation parameters were calculated. The identity of the donor atom of the ambidentate ligands in the rate-determining processes, kII, is considered and correlations found between ΔH ‡kII, the enthalpy of activation, and ΔS ‡kII, the entropy of activation, respectively, the Mulliken population on the donor atom, and the energy of the highest occupied molecular orbital (HOMO) with σ symmetry. The parameters ΔH ‡kII is strongly dependent on the HOMO energy, and more weakly dependent on the electron density on the donor atom, while the converse is true for ΔS ‡kII.