The rheological and mechanical properties of a polystyrene-polyisobutylene-polystyrene (SIBS) block copolymer containing 30 wt% polystyrene (PS) and its blends with PS (SIBS/PS) were investigated. Atomic Force Microscopy (AFM) was used to visualize the nanostructured phase morphology of the SIBS, which is responsible for the mechanical strength of this thermoplastic rubber. The order-disorder transition (ODT) for the SIBS block copolymer was found to be above 250degreesC. SIBS/PS blends with 10-30 wt% PS showed improved moduli and tensile strengths. Blends containing up to 40 wt% PS behaved as thermoplastic elastomers. In the region of linear viscoelasticity the blends revealed pronounced non-Newtonian behavior and enhanced elasticity. This paper also reports the role of this styrenic block copolymer in the impact modification of PS.