The effectiveness of toughening brittle glassy polymers such as polystyrene (PS) through deformation-induced plasticization by low molecular weight diluents of polybutadiene (PB) was amply demonstrated in earlier studies. In those applications, surface-initiated crazes of unusual growth kinetics and stability could produce effective toughening in sheet samples of millimeter thicknesses, but would have been ineffective in more massive parts where crazes could not be initiated in the interiors to promote a plastic response of the entire volume. This shortcoming has now been rectified through the development of ternary blends incorporating into the previous PS/PB blends a critical small volume fraction of ABS- or HIPS-type composite particles that serve to initiate crazes throughout the volume. Thus, we demonstrated in the present study that incorporation of 10% commerical ABS or 20% commercial HIPS into the most effective PS/PB-3K blend results in tensile toughnesses equal to or exceeding those of commercial ABS or HIPS in full concentration.