The proportions of pyrolysis and hydrolysis in poly(ethylene terephthalate) (PET) degradation were determined from the decomposition of PET in O-18-labeled steam, producing labeled and unlabeled terephthalic acid. At a 100 vol % steam concentration at 400 degrees C, 74% of the ester bonds were pyrolyzed and 26% were hydrolyzed, as determined by gas chromatography-mass spectrometry. In addition, thermogravimetric analysis revealed the activation energy to be 199 kJ mol(-1) under pyrolytic conditions, decreasing with increasing steam concentration, and reaching a minimum of 132 kJ mol(-1) at 75 vol %.