The ageing properties of Styrene-Butadiene-Styrene (SBS) polymer modified bitumens were evaluated using Dynamic Mechanical Analysis (DMA), Gel Permeation Chromatography (GPC) and Fourier Transform InfraRed (FTIR) spectroscopy. The binders were aged by means of the Thin Film Oven Test (TFOT) and the Rolling Thin Film Oven Test (RTFOT), respectively. It was observed that ageing resulted in degradation of the SBS polymer (decrease in the polymer molecular weight) and oxidation of the bitumen (increase in the content of polar oxygen-containing molecules and increase in bitumen molecular weight). The polymer was observed to resist formation of sulphoxides. Changes in the rheological properties of aged-modified binders were dependent on a combined effect of bitumen oxidation and polymer degradation, which varied with bitumen source/grade and polymer type/content. In all cases, the aged modified binders showed better rheological properties than aged base bitumens. The study also indicated that the ageing index obtained using DMA was largely influenced by temperature and frequency. This parameter may be applied for evaluating the base bitumens and modified binders with a low polymer content (3% in this study), but not for modified binders with a high (6%) polymer content.