SBS styrene-butadiene-styrene elasts in bitumen/oil fraction mixtures have been investigated with respect to changes in the molecular mass in the SBS fraction. This has been done at temperatures around 200 °C. Gel Permeability Chromatography (GPC) has been used which has resulted in a method for quantitative measurements of SBS degradation. It has also been possible to observe how the molecular weight distribution in the final product is changed. Trials to measure changes in the mechanical properties have also been done by means of Thermo Mechanical Analysis (TMA). These measurements have been compared with the GPC analyses.