Block copolymers can be good compatibilizers in polymer blends, if tailored carefully. However, they are produced by anionic polymerization which is not a versatile technique and, moreover, is expensive. Suitable alternatives are block-graft copolymers, which can be prepared by radical grafting from readily available block copolymers, in particular from SbB (S, styrene; B, butadiene). The efficiency of two block-graft copolymers, SbBgCHMA and SbBgMMA (CHMA, cyclohexylmethacrylate; MMA, methyl methacrylate), as compatibilizers in PS/PVC blends of polystyrene (PS) and poly(vinyl chloride) (PVC) was investigated by electron microscopy. Both copolymers refined the coarse morphologies of the PS/PVC blends excellently, covering the interfaces between the phase domains of PS and PVC with monolayers. This is in accord with the commonly assumed model of the compatibilizer effect of block copolymers. However, the phase structures were, in fact, even finer than predicted by the model : perfectly compatibilized phase domains were torn apart during mixing in the kneader, whereby the morphology was further refined.