Abstract Polyethylene is nowadays used in many applications with lifetimes of several decades. To predict the lifetime artificial aging is widely used, but only little is known of the chemical change of the polymer chains itself. We have used different types of unstabilized polyethylene and aged them in water at elevated temperature and a high pressure of oxygen to accelerate the natural aging processes and analyzed the materials by Py-GC-MS, infrared spectroscopy and size exclusion chromatography. With pyrolysis 2-oxoalkanes and 2-oxoalkenes as well as carboxylic acids were identified as the degradation products. Thermally assisted hydrolysis and methylation was successfully applied to assess the degree of oxidation, especially targeting the more polar compounds. With this technique alkane dioic acids have been identified as valuable marker compounds for the oxidative degradation of polyethylene. The mechanism of formation of fatty acids in thermally assisted hydrolysis and methylation has been elucidated using two different alkanones. With size exclusion chromatography it could be shown that the oxidation occurs randomly along the polymer chain and that the final degradation products are in the range of a few thousand g mol-1, irrespective of the original molar mass.