The heat of base-catalyzed alcoholysis of poly(ethylene terephthalate) (PET) and poly(ethylene 2,6-naphthalenedicarboxylate) (PEN) as well as several related aromatic and aliphatic esters has been measured using a high-pressure calorimeter. The process is essentially thermoneutral, acid heats of CH3ONa-catalyzed methanolysis of PET and PEN are +1.8+/-0.6 and +9.3+/-0.2 kJ/mol-carboxyl group, respectively. Furthermore, the heats of Zn(OCOCH3)(2)-catalyzed alcoholysis of PET with ethylene glycol and benzyl alcohol are also calculated as +5.8+/-0.2 and +6.6+/-0.8 kJ/mol-carboxyl group, respectively. These values are comparable to those for ethyl esters of p-substituted benzoic acids and propionic acid.