Development of morphological structures during phase separation induced by radical polymerization of 2-chlorostyrene in the presence of polystyrene was studied with an electron microscope. No stirring of the mixture was made. Domain structure changed significantly with initial monomer composition within a narrow range. Salami domains similar to those observed in high-impact polystyrene (HIPS) formed at certain initial compositions, notwithstanding that production of graft polymers, which was known to play an essential role of the salami structure formation in HIPS, was negligible in this system. An explanation for the mechanism of salami pattern formation was proposed. (C) 2003 Elsevier Science Ltd. All rights reserved.