The effect of pressures up to 3 kbar on the rate of the acid-catalyzed hydrolysis of methyl, ethyl, andt-butyl acetates in dilute aqueous acid and of ethyl acetate in concentrated hydrochloric acid has been measured. The volume of activation fort-butyl acetate is zero within experimental error, showing that the mechanism is unimolecular. Those for methyl and ethyl acetates are near –9 cm3mole−1in both dilute and concentrated acid. We deduce from this that the mechanism is the same in 9.2-Mhydrochloric acid as in dilute acid, that the transition state is not highly polar, and that if the proton in the reactive protonated ester is on the carbonyl oxygen then the attacking water molecule adds, and if the proton is on the ether oxygen then the attacking water molecule substitutes.