High-impact-modified polystyrene (HIPS) is made by thermal or radical polymerization of styrene containing dissolved polybutadiene (PB). The reaction leads, intermediately, to blends of yet ungrafted PB, of the homopolymer PS, and of graft copolymers PBgS varying in the number of PS grafts per PB chain. The polymerization induces a phase separation and a phase inversion which results in the well-known "salami" morphology of HIPS. To elucidate the mechanism producing this morphology, the polymerization of styrene in PB/styrene mixtures was studied kinetically and morphologically, in toluene solution and in the bulk. Besides the conventional techniques of polymerization kinetics, electron microscopy was employed to examine the PS/PB/PBgS blends that are formed during the polymerization. The electron micrographs reflect sensitively the composition of these blends, the architecture of the PBgS graft copolymers, and the miscibility of PBgS with PS and PB. The blends were isolated from the polymerizing system by two methods, i.e. (i) evaporation of styrene and (ii) dissolution and film casting. Method i preserves the in-situ morphology of the polymerizing system, while method ii leads to a thermodynamically controlled morphology. Pairwise comparison of these two types of morphology reveals that HIPS owes its salami structures to the fact that PBgS chains with two grafts or more can solubilize the homopolymer PS while PBgS chains with only one graft cannot.