The kinetics of base hydrolysis of a series of (halogeno)(amine)cobalt(III) complexes, cis-[(en)₂Co(B)X]²⁺(B = NH₃, CH₃NH₂, CH₃CH₂NH₂, C₆H₅CH₂NH₂, C₆H₁₁NH₂, imidazole, N-methylimidazole, benzimidazole, CH₃CH(OH)CH₂NH₂, X = Cl, Br) have been investigated in methanol–water media (0–90 vol.% MeOH) at 15.0 ⩽t/°C ⩽ 40.0 (0.02 mol dm^–3 NaOH). The relative second-order rate constants at ionic strength, I= 0, [(k^OH₂)^{0 s}/k^{0 w}₂], generally increased non-linearly with increasing mole fraction (X_MeOH) of MeOH; the effect was sensitive to the bulkiness and hydrophobicity of the non-labile amine ligands. The solvent effects on rate have been interpreted in terms of the role of solvent structure and preferential solvation of the initial state and transition state, presumably due to the hydrophobic interaction. This is evident from the linear plot of log(k^OH₂)^{0 s}versusV°^1/3 at 25 °C where V° is the standard partial molar volume of the complex ion in aqueous medium at 25 °C. The activation enthalpies and entropies also varied non-linearly with X_MeOH and the observed extrema in the plots of ΔH^#(ΔS^#)vs. X_MeOH presumably reflected the solvent structural effect on the solvation component of these thermodynamic parameters.