The thermooxidative degradation of poly(oxymethylene) copolymer (CPOM-Y) powder was studied in air in 150degreesC. The effect of the sequence distribution of degraded poly(oxymethylene) (POM) samples on the thermal decomposition behavior yeas investigated with H-1-NMR and gas chromatography/mass spectrometry. The results showed that the degradation process of CPOM-Y could be divided into three stages with a gradually increasing degradation rate. The change in the sequence molar fractions agreed with the fact that the ethylene oxide (EO) units had higher thermal stability, and the degradation of POM was due to the decomposition of formal units. At the beginning, CPOM-Y tended to split off formaldehyde, starting at the chain ends, some of which were not ended by EO units. In stage 2, the thermooxidation of POM occurred in the amorphous phase. In stage 3, no obvious rules for the changes in the sequence contents were obtained from NMR results, and this indicated further random chain scission and unzipping occurring heavily in the crystalline bulk. (C) 2004 Wiley Periodicals, Inc.