The compatibilizing efficiency of epoxidized styrene-butadiene block copolymers [E(S-b-B)] of varying architecture for the immiscible polystyrene (PS)/chlorinated polymer (CP) blends [poly(vinyl chloride), chlorinated polyethene] was examined. The copolymers were a diblock (ESB), a three-arm star (ESB), and a random ESBR. The techniques applied were dynamic mechanical analysis and tensile testing of specimens prepared from solvent casting. Compatibilizing efficiency was found to depend on the degree of epoxidation of the butadiene unit in the copolymer, which in turn relates to the degree of chlorination of the CP, and also on the CP/PS ratio in the ternary blend and the molecular architecture of the E(S-b-B) compatibilizer. Other things being equal, this efficiency varies as : ESB>(ESB)3>ESBR; thus it parallels the behaviour of the E(S-b-B) towards the CP in the binary blends. The same holds true for the degree of epoxidation of the E(S-b-B). This is in agreement with recent theoretical compatibilizing efficiency predictions and relates to the relative strength of the interaction parameters among blend constituents. Use of appropriate values for these parameters allowed the determination of the spinodal, approximating successfully the immiscibility gap of the ternary. This procedure could only be applied to the PS/ESBR/CP system, since ESBR, unlike the other E(S-b-B) copolymers, is not microphase separated.