We have taken blends of a bitumen with three homopolymers that produce systems softer than the bitumen, and to each have added small quantities of different semicrystalline polyolefins. The intention is to discover whether these Will form an extensive network system within a blend with a low T-g in which crystallites create the cross-links, as may happen with the isotactic polypropylene present in some atactic polypropylene systems. Such thermoplastic cross-links might provide a useful feature for certain uses, such as in built-up roofing membranes. These were found only within one blend based upon the polybutadiene, in two blends based upon chlorinated polyethylene, but in none based on the Lycra(R) whose blends with LLDPEs and the HDPE flowed below the crystal melting temperature. The E" plots indicate the presence of many loss processes within the blends at low temperatures: the tan 6 plots show other processes active up to 200degreesC within the soft matter. The behavior we discovered suggests that the properties of binary polymer blends are not associated solely with the components, but that, when the material is stiff or after heating, certain polymer-bitumen interactions may also be significant, and are sensitive to slight differences of structure between LLDPEs, even when present at the 3.4% level.