Hydrolysis of p-methylphenyl trichloroacetate in water-acetonitrile mixtures was studied as a function of water concentration in the range 5.5-55.5 M. The proton inventory technique, in H 2 O-D 2 O mixtures, shows, for a value of D atom fraction in the solvent n ¼ 0.5, deviations from the expected value (for a reaction with one proton being transferred) of 7.5 and 12.3%, for experiments in the presence of 16.6 and 33.3 M L 2 O (L ¼ H or D), respectively. Theoretical treatment of the data obtained at [L 2 O] ¼ 16.6 M using the Gross-Butler equation are consistent with a cyclic transition-state structure with three protons involved. Conversely, similar experiments in the presence of [L 2 O] ¼ 33.3 M show that multiple water molecules are involved in the transition state of the reaction.