AbstractThe kinetics of the solvolysis of the hydrophobic ion, Co(4‐t‐Bupy)4Cl2+, have been followed in mixtures of water with co‐solvents having a hydrophilic tendency, ethane‐1,2‐diol and 2‐methoxyethanol. The variation of In(rate constant) with the reciprocal of the dielectric constant is nonlinear for both co‐solvents. The enthalpy and the entropy of activation are rather insensitive to changes in the solvent composition in both mixtures, but low maxima may exist at mol fractions of co‐solvent Ca. 0.10–0.20. The application of a Gibbs energy cycle to the process of the initial state going to the transition state suggest that, in water‐rich conditions, the increase in the stability of the emergent solvated CoIIIion in the transition state relative to the increase in the stability of Co(4‐t‐Bupy)4Cl2+in the initial state as the co‐solvent content rises is greater for 2‐methoxyethanol than for ethane‐1,2‐di