Abstract The rates of hydrolysis of phenylchloroformate were measured over the temperature range 30–55 °C in water-tetrahydrofuran and water-dioxane mixtures of different compositions. The influence of solvent variation on reaction rate was examined in terms of changes in the activation parameters. The activation parameters ΔH★ and ΔS★ for the solvolysis reaction showed minima at a water mole fraction of about 0.5 in water-tetrahydrofuran solvent mixtures, whereas these parameters increased continuously with increasing concentration of water in water-dioxane solvent mixtures. The results are discussed in terms of the polarity of the medium and the change in the solvent structure.