The reaction rates for the esterification of a substituted phenylacetic acid by phase-transfer catalysis were measured using a constant interfacial area cell. Experiments were performed at different concentrations of aqueous-phase reactant, phase-transfer catalyst, NaOH, and organic-phase reactant. It was shown that the stirring significantly affected the reaction rates. An attempt was made to express the reaction rate as a function of the concentrations of reactive species. The effect of temperature on the reaction was also studied, and the activation energies were evaluated. A possible mechanism was proposed and discussed based on a mixed Makosza and modified interfacial mechanisms.