To investigate the effect of solvent structure on the first-order solvolysis of the trans-dichlorotetrapyridinecobalt(III) ion, rates have been measured in.mixtures of water with isopropyl alcohol as the physical properties of these mixtures show that the addition of isopropyl alcohol to water in water-rich conditions is very effective in inducing the formation of structure in the solvent. A linear plot of log (rate constant) against the Grunwald–Winstein Y factor confirms that the solvolysis is an SN1-type process with considerable extension of the metal-chloride bond in the transition state. A non-linear plot of log (rate constant) against reciprocal of the dielectric constant shows that changes in solvent structure with composition are important in determining the variation of rate constant and this is confirmed by the observation of a maximum in the variation of both the enthalpy and entropy of activation in the region where the physical properties of the mixture indicate a maximum in the structure formation. The application of a free-energy cycle shows that the effect of structure is greater in the transition state than in the initial state.