The effect of rubber phase content on notched Izod for polystyrene (PS) and polybutadiene (PB) blends compatibilized with PS-PB diblock and PS-PB-PS triblock copolymers has been studied. The effectiveness of various PS-PB-PS triblock copolymers in improving the impact strength of PS/PB blends has been studied. The effect of blending PS-PB diblock with commercial HIPS has been explored. Studies were done for bulk blends produced by compositional quenching. It was observed that a minimum rubber phase volume of 7% was required for significant impact modification. This volume fraction corresponds to a interparticle distance of one particle diameter. The increase in impact strength was attributed to stress field overlap. HIPS blends showed no synergistic impact behavior, previously attributed to particle size bimodality. Atomic Force Mllicroscope studies performed on fracture surfaces showed that energy dissipation is not associated with creation of surface area. A notched Izod of 5.6 ft.-lbf./in. combined with a modulus of 2.17 GPa was obtained using a medium molecular weight triblock, having a block molecular weight of 40,000-24,000-40,000. The triblock results were marginally worse than the best results obtained with diblocks(27) (Mathur and Nauman, J Appl Polym Sci, 1999), but were significantly better than results obtained with multiblock and random copolymers(1) (Cavanaugh et al., Polymer, 1998).