Thermo-oxidative ageing of an unfilled and unstabilized peroxide-cured nitrile rubber containing 33wt% acrylonitrile was investigated for ageing conditions at 100 and 120°C up to 21 days. Oxygen consumption was measured by a self-developed test setup based on respirometry to get a closer look on the adsorption or rather absorption behavior. FTIR spectroscopy was performed to analyze spatial chemical changes in the elastomer as function of ageing time and temperature as well as the development of oxygen containing functional groups. Carbonyl nitrile ratios over the cross section of the sample showed a highly heterogeneous oxidation behavior at 120°C and less oxidation with more homogenous profiles at 100°C. Oxidation was concentrated to the edge layer and effects of diffusion-limited oxidation were observed. Oxygen consumption measurements underlined the findings by showing an unexpected harsh decline of absorption at 120°C which indicated a restrained absorption and diffusion behavior. An enhanced model was used to simulate the heterogeneous oxidation and to demonstrate the influence of contributing factors on the oxidation behavior. The approach is based on a modified reaction-diffusion equation which describes the degree of oxidation with two ageing parameters representing the level of chain breaking and crosslink building reactions. The investigation shows that adsorption, diffusion and reaction mechanisms are highly temperature dependent and each has to be considered both separately and including their interactions in order to get a deeper understanding of the thermo-oxidative ageing of elastomers.