Kinetics of the hydrolysis of 3-(2'-chloro-6'-fluorobenzyl)isocoumarin (1), 3-(4'-fluorophenyl)isocoumarin (3), 3-(2'-fluorophenyl)isocoumarin (5), 3-(2'-chlorophenyl)isocoumarin (7), and 3-(4'-chlorophenyl)isocoumarin (9) were studied spectrophotometrically in the presence of aqueous solution of potassium hydroxide. The rates of hydrolyses for these isocoumarins were compared. The data for isocoumarin (1) were further investigated to probe the reaction mechanism. The dependence of pseudo-first order rate constant k(obs) on hydroxide ion concentration is fitted by the expression k(obs) = k(o) + k(1) [OH] + k(2) [OH](2), showing the formation of monoanionic and dianionic tetrahedral intermediates during the course of reaction. The magnitude of ko shows significant participation of solvent in the reaction mechanism.