Dynamic mechanical thermal analysis (DMTA) and oscillatory flow measurements are carried out to investigate the rheological properties of 70/30 blends of a bitumen with metallocene catalyzed atactic polypropylenes. DMTA results indicate that saturated liquid resins of the bitumen act as a plasticizer on polypropylene. The glass transition temperature of polypropylene decreases, and the pure bitumen broad relaxation disappears, which signifies that a polypropylene-saturated resin mixture constitutes the continuous phase. Such results are not obtained when a conventional atactic polypropylene is used in the blend. Oscillatory flow measurements show a viscosity reduction of the blends with respect to pure polypropylene, beyond what could be expected from a simple additive rule, reflecting the plasticization of polypropylene. One of the blends displays a viscoplastic behavior at low frequency, which is attributed to the presence of small crystals, formed on cooling from the mixing temperature. Some viscoelastic properties related to the use of these materials as roofing membranes are analyzed. (C) 2004 Society of Plastics Engineers.