The Row-induced morphology of ternary blends containing poly(styrene-ro-acrylonitrile) (SAN) random copolymer as major phase. polystyrene (PS) as minor phase, and poly(styrene-b-methylmethacrylate) (PS-b-PMMA) block copolymer as emulsifier is investigated as a function of the acrylonitrile content of the SAN copolymer. In this way. the degree of exothermic interaction and swelling power between SAN and PMMA block is varied systematically. Consistent with previous work, a well-emulsified morphology is produced when the swelling tendency of the block segment in the major phase (S-out) is comparable to that in the minor phase (S-int) (0.4 less than or equal to Sout/Sint less than or equal to 2.5). Outside this window, emulsification failure occurs. We further investigate the influence of S-out/S-int on the formation of phase-inverted (PIN) morphology, induced by lowering the viscosity of the disperse phase. We demonstrate that a decrease in swelling power assists the tendency of the minor phase to be continuous, i.e. the formation of PIN structure is possible at a lower weight fraction of the minor component. (C) 2001 Elsevier Science Ltd. All rights reserved.