Poly(butylene terephthalate) (PBT), and its oligomer, were doped with the spin-trapping reagent, 2-methyl-2-nitrosopropane (MNP), by a solution-casting method in order to detect the short-lived radical intermediates during thermal degradation. In the case of PBT, two types of spin adduct were observed by electron spin resonance (ESR) spectroscopy upon stepwise heating from room temperature to 200 degrees C and during annealing at 130 degrees C; the ESR spectrum exhibited an isotropic three-line signal, with a hyperfine coupling constant (hfcc) a(N) approximate to 0.8 mT, as well as an anisotropic component. These adducts were present on the order of 10(-5) mol/L. The isotropic component was assigned to arise uniquely from the benzoyl radical, (CO)-C-center dot-phi The anisotropic component was carefully investigated by ESR spectroscopy and high-performance liquid chromatography-electrospray ionization-mass spectrometry (HPLC-ESI-MS), using low-molecular-weight model compounds such as n-butyl benzoate (BB), di-n-butyl terephthalate (DBT), and deuterated DBT (d-DBT). The newly observed ESR components, six lines for DBT and nine lines for d-DBT, were assigned to the alpha-carbon radicals,-O-(CH)-C-center dot- and -O-(CD)-C-center dot-, respectively. The existence of these spin adducts was also confirmed directly by MS analysis (as protonated ions). We conclude that the thermal degradation of PBT begins by hydrogen abstraction from the alpha-methylene group of PBT at low temperature (110 degrees C), with subsequent main chain decomposition through beta-scission of the ester linkages.