AbstractThis article investigates the room temperature demixing of oil-in-oil emulsions containing styrene (St), polybutadiene (PB), a St-butadiene star block copolymer (BC), and two polystyrene (PS) samples of different molecular weights and is a contribution toward a better understanding of the stability/instability of the reaction mixture in a bulk high-impact polystyrene (HIPS) process close to the phase inversion. Twelve bulk prepolymerizations of St in the presence of PB were emulated, at 10%, 15%, and 20% conversion; and with constant grafting efficiencies. All the blends contained 6% in weight of butadiene units. After stirring the blends for 24 h, the decantation demixing process was monitored along 30 days, with daily measurement of the interface levels after appearance of a clear interface. For some of the isolated phases, their unswollen morphologies were observed by transmission electron microscopy. All the isolated phases exhibited macrophase separation into homopolymer- and copolymer-rich macrodomains with lamellar microdomains. The BC showed a greater affinity toward the PS-rich phase. The separation of an independent BC-rich phase in the blends containing the high molar mass PS and at high grafting efficiencies, modifies the idea of the graft- or BC molecules located at the interface of large PS-rich and PB-rich phases.