© 2017 Elsevier LtdThe effect of pro-degradant manganese (Mn) stearate concentration on the oxidative abiotic degradation of high-density polyethylene (HDPE) was investigated in thermo-oxidative tests at three temperatures (60, 70 and 80 °C) over time. Degradation monitoring was carried out, by determining carbonyl indices (CI), using infrared spectroscopy (IR), and by assessing molar masses and their distributions, using size exclusion chromatography (SEC) and tensile tests to assess mechanical properties. Deconvolution analyses of the absorbance bands obtained in the carbonyl region were performed to quantify the functional groups originated from degradation. Similar levels of degradation (equivalent molar mass and oxidation values) were obtained regardless of the investigated manganese concentration (50, 200 and 400 ppm). However, the results indicate that increasing Mn concentration may reduce the induction time for carbonyl formation. Deconvolution of the carbonyl region in IR spectra (between 1800 and 1650 cm−1) indicates that the addition of Mn results in increase in the concentration of oxidized functional groups (ketones, carboxylic acids, lactones, etc.) and that relationship between these groups varies depending on whether manganese stearate is present or not.