Bulk blends were used to study the effectiveness of various PS-PB diblock copolymers in improving the impact strength of PS/PB blends. The blends were produced by compositional quenching, and further ripened to control particle size and the diblock concentration at the homopolymer interface. It was found that an optimal molecular weight of diblock copolymer exists for the maximum notched Izod. Low molecular weight diblocks provided insufficient adhesion due to lack of entanglements, and high molecular weight diblocks provided insufficient interfacial concentration due to stearic crowding. The maximum notched Izod, for 23% rubber phase volume, was achieved with 1-mu particles, a copolymer surface density of 0.17 chains/nm(2), some crosslinking, and a medium molecular weight, tapered diblock, M-w = 154,000, having a 30% PS content. The combination of high impact strength, 6.2 ft.-lbf./in., and high modulus, 2.2 GPa, significantly exceeds that of commercially available super high-impact polystyrene.