The ageing behaviour of Eurobitum bituminised radioactive waste was studied by comparing the characteristics of 25 years old radioactive Eurobitum with those of 25 years old non-radioactive Eurobitum samples that were heated or gamma-irradiated in the presence or absence of oxygen. Chemical changes in the bitumen structure were followed in the mid-infrared region with Attenuated Total Reflectance Fourier Transform InfraRed Spectroscopy (ATR-FTIR) by measuring the evolution of the band heights at 1700 cm-1 (C=O functional groups) and 1600 cm-1 (C=C double bonds). Oxidation of bitumen in the presence of oxygen results in a distinct increase of both the number of C=O and C=C functionalities, with a positive linear relationship existing between the two groups. The production of C=O functional groups seems to promote the generation of C=C double bonds. Heating at 130 °C is much more efficient to oxidise the bitumen than gamma irradiation at low to moderate dose rates (20–140 Gy/h). As the oxygen concentration decreases, for instance by diffusion limitation deeper inside the bitumen, the number of C=O and C=C functionalities formed per unit of time decreases. A similar behaviour was observed for 25 years old radioactive Eurobitum.