Depolymerization of poly(ethylene terephthalate) (PET) to terephthalic acid (TPA) and ethylene glycol (EG) and poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) to 2,6-naphthalene dicarboxylic acid (2,6-NPA) and EG in water at high temperature (> 523 K) was investigated for the purpose of monomer recycling. In case of the depolymerization of PET in water, the yield of TPA increased to 90% with increasing reaction temperature up to 693 K while the maximum yield of EG achieved was 60% at 573 K. For PEN depolymerization, the yield of 2,6-NPA also increased to 97% with increasing reaction temperature up to 693 K and the maximum yield of EG obtained was 60% at 573 K. The higher yield of 2,6-NPA than that of TPA was ascribed to the higher stability of 2,6-NPA than that of TPA. The lower yields of EG than the corresponding monomers (TPA and 2,6-NPA) in the decomposition of the two kinds of polyesters were caused by the catalytic dehydroxylation of EG by protons derived from dicarboxylic acids. (c) 2005 Elsevier B.V. All rights reserved.