The thermal oxidation of unstabilised POM homopolymer has been studied at 90, 110, and 130 degrees C under various oxygen pressures ranging from 0 to 2.0 MPa by gravimetry and IR spectrometry. This latter reveals essentially the formation of formate (F) groups and alcohols in smaller quantities. The formate growth and mass loss rates increase pseudo hyperbolically with oxygen pressure, the asymptotic level being reached at pressures up to 0.5 MPa. These results lead to propose a mechanistic scheme of radical chain oxidation in which depolymerization is initiated by hydroperoxide unimolecular decomposition, one formate and one chain scission being formed per decomposition event whereas the kinetic chain length of the depolymerization zip is of the order of 100. A kinetic model derived from the mechanistic scheme, taking into account substrate consumption, simulates correctly the shape of kinetic curves and the effect of temperature and oxygen pressure. (c) 2007 Wiley Periodicals, Inc.