Microstructural transformation of a poly(styrene-b-butadiene-b-styrene), SBS, triblock copolymer blended with asphalt was studied as the asphalt composition was varied from 0 wt% to 96 wt%. Transmission electron microscopy (TEM), dynamic mechanical spectrometry (DMS), and differential scanning calorimetry (DSC) were used. The blends were made in batch mixers at 200 degrees C, or by solution casting from a nonselective solvent (trichloroethane) at similar to 28 degrees C. Asphalt partially solubilizes the polybutadene (PB) midblock of the SBS producing saturated PB microdomains along with macrodomains of asphalt. When the asphalt concentration was varied from 10 to 90 wt%, the asphalt phase separated into a variable number of large domains, while the SBS-rich regions formed a continuous matrix. Networks of SBS-rich regions were observed at low magnification; these are referred to as macro-networks. At higher magnification, networks that are stabilized by polysty rene (PS) microdomains (denoted micro-networks) are also formed. The presence of a macro-network is also confirmed by stress relaxation tests. The macro-network broke down into microgel-like structures when the asphalt composition exceeded 90 wt%. Examination of the interior of the SBS-rich regions showed that the shape of the PS microdomains transformed from short cylinders to lamellae, hexagonally perforated lamellae (HPL), back to lamellae, short cylinders, and finally to spheres. DMS and DSC indicate a systematic increase in the PB glass transition temperature (T-g) and negligible change in the T-g of PS as the asphalt content increases. Triblock copolymers that can form a macro-network at low concentration will be more desirable for highway pavement modification.