Brittle isotactic polystyrene has been toughened by the presence of emulsified rubber particles. Semicrystalline diblock copolymers of isotactic polystyrene and polybutadiene functioned successfully as emulsifying agents, as revealed by the results of scanning and transmission electron microscopy. Semiquantitative measures of the toughness of the various materials were obtained via a home-built micromechanical testing device. The minimum amount of copolymer required to achieve emulsified morphologies and significant toughness depended on the thermal history of the material being tested.