The isotopic acyl exchange reaction between various phenyl acetates and acetic acid was studied in pyridine solution. No steric effect on the exchange rate was observed in the case of o-substituted phenyl acetate, but the polarity of the substituents existing on the benzene ring afforded remarkable effects on the exchange rate. The electron-attracting group on the benzene ring promoted the exchange reaction. Kinetic data showed that the reaction center of the ester was the carbonyl carbon atom. The nucleophilic reactant which was formed by the preliminary reaction attacked the reaction center in the ester, followed by the bond migration which was the true exchange step and was considered to be rate-determining step. The substituents on the benzene ring gave the polar effect on the rate-determining step.