Rates of solvolysis of the anion [Co(CN)₅Cl]^3– have been investigated in water and in a range of water–methanol mixtures over a range of temperatures. The variations of the rate constants and the enthalpy and entropy of activation with solvent composition are compared with changes in the physical properties of water–methanol mixtures. The application of a free-energy cycle to the dissociative process of the initial state [Co(CN)Cl^3–₅] going to the transition state [Co(CN)^2–₅⋯Cl^–] in water and in the mixtures suggests that changes in solvent structure in going from water into the mixture have a stabilising influence on the emergent Co(CN)^2–₅ ion in the transition state greater than that on the Co(CN)Cl^3–₅ ion in the initial state. These variations are compared with the variations of similar properties found earlier for the solvolyses of complex cations of Co³⁺ in mixtures of methanol and of other alcohols with water.