Abstract The character of the thermal decomposition of polyoxymethylene, acetylated polyoxymethylene and Delrin in air and in nitrogen has been studied by thermogravimetric analysis. Two peaks have been observed in each atmosphere in the derivative thermogravimetric analysis curves for these samples. It has been confirmed that the feature of doublet peaks is related to the concentration of oxygen during the decomposition and, thus, that the first stage is due to the chain depolymerization as well as to the oxidative decomposition, and that the second stage is to be attributed to the random degradation of a stable fragment formed during the first stage decomposition. An increase of carbonyl groups in the polymer molecule with the degradation was confirmed by infrared spectroscopy.