The efficiency of structurally different copolymers as compatibilizers for the homopolymer blend PVC/PS (polyvinylchloride/polystyrene) was tested. As expected, a diblock copolymer SbMMA of styrene and methylmethacrylate, with a MMA block twice as long as the styrene block, did well, refining the phase morphology of PVC/PS drastically. But a random copolymer ScoMMA with the same monomer composition was efficient as well. Particularly interesting are block-graft copolymers made by grafting MMA or a random copolymer CHMAcoMA (CHMA: cyclohexylmethacrylate, MA: methylacrylate) onto a diblock copolymer SbB of styrene and butadiene. With these block-graft copolymers, a mechanism of compatibilization in two stages was observed that leads to finer phase structures than theoretically expected.