We investigated the thermal degradation of poly(butylene terephthalate)-co-poly(ethylene oxide) (PBT-co-PEO), a thermoplastic poly(ether-ester) elastomer (TPEE), through proton nuclear magnetic resonance (NMR) spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), gel permeation chromatography (GPC), and spin-trapping electron spin resonance (ESR) analysis to detect radical intermediates present during degradation. Three kinds of PBT-co-PEO with different component ratios of PBT/PEO were synthesized. The thermal degradation process of PBT-co-PEO was divided into four weight-loss stages. In the first stage from room temperature to 120 degrees C, we confirmed the production of two radical intermediates, O-(CH)-C-center dot-CH2 and (CH2)-C-center dot-, by the spin-trapping method. This suggested that the initial decomposition occurred at the OCH2-CH2O bond in the PEO units. Upon annealing at 120 degrees C, PBT-co-PEO showed a small degree of random degradation, while higher PBT contents induced gelation and the production of the characteristic oligomer PBT PEO PBT. The gelation was attributed to cross-linking between two O-(CH)-C-center dot-CH2 moieties of PBT. In the second stage from 120 to 340 degrees C, a large thermo-oxidative degradation of the PEO segment occurred, accompanied by an increase in the radical amount of spin adducts. In the third stage, the thermo-oxidative degradation of PBT units was observed. The radical intermediates were thus shown to be a primary factor of the thermal degradation characteristics of TPEE.