End groups after the thermal degradation of poly(ethylene terephthalate) (PET) and its cyclohexanedimethanol (CHDM) copolymer were characterized with H-1 NMR. Thermally degraded polymers were obtained by heat treatment at 290 degreesC. For the PET homopolymer, a vinyl end group appeared, which resulted from thermal cis-beta -elimination. For the CHDM copolymer, in addition to a vinyl end group, methylcyclohexene and cyclovinylidene end groups originating from CHDM were formed. The assignment of the H-1 NMR spectrum was performed with information from C-13 NMR and gas chromatography-mass spectrometry. The total amounts of unsaturated species measured by NMR were compared with those estimated by bromination titration. There was good agreement between the values obtained by the two methods, indicating that all the major unsaturated species were accounted for. The mechanism of the formation of the unsaturated end groups was investigated. We suggest, on the basis of the NMR measurements, that the methylcyclohexene and cyclovinylidene groups originating from CHDM were formed by thermal cis-beta -elimination as for the PET homopolymer. (C) 2001 John Wiley & Sons, Inc.