Two styrene-butadiene triblock copolymers differing in the length of their styrene blocks (40S-60B-40S and 10S-60B-10S) were used as compatibilizers for PS/PB (4/1) and PS/PP (4/1) blends. The supramolecular structure of the copolymers determined by small-angle X-my scattering (SAXS), morphology of the blends using transmission electron microscopy (TEM), and their tensile impact strength were chosen as criteria of the compatibilization. efficiency of the copolymers used. Different mechanisms of compatibilization. for "symmetrical" system (PS/PB/SBS) and "asymmetrical" system (PS/PP/SBS) were proved. While for the PS/PB blend, the 40S-60B-40S copolymer proved to be a good compatibilizer, for the PS/PP blend, surprisingly, the 10S-60B-10S copolymer is more efficient.