Abstract Blends of bitumen with 3–10% of a styrene–butadiene–styrene (SBS) block copolymer were investigated by means of modulated differential scanning calorimetry (MDSC) in an effort to better understand the miscibility and structure of the blends, along with the composition of the mixed phase(s). This relied on the measurements of the glass transition temperatures (Tg) in bitumen and SBS in their blended and unblended states. In the unblended state, bitumen showed four Tg's, and SBS showed two. In the blends, a new Tg arose from a phase of mixed composition, which contained polybutadiene (PB) segments and about 30% of the maltenes. The blends also showed anti-plasticization as a shift of the Tg's from the paraffins in bitumen and the PB block in SBS moved away from each other. The results indicate that the PB block has good interactions with bitumen, but that the polystyrene (PS) block does not.