The compatibilization efficiency of two styrene-butadiene-styrene triblock copolymers with short (SB1) and long (SB2) styrene blocks was studied in polystyrene (PS)-polypropylene (PP) blends of composition 20, 50, and 80 wt % PS. The supramolecular structure of the blends was determined by small-angle X-ray scattering, and the morphology was studied with transmission electron microscopy and scanning electron microscopy. Structural changes in both the uncompatibilized and compatibilized blends were correlated with the values of tensile impact strength of these blends. Even though the compatibilization mechanisms were different in blends with SB1 and SB2, the addition of the block copolymers to the PS-PP 4/1 and PS-PP 1/4 blends led to similar structures and improved the mechanical properties in the same way. These block copolymers had a very slight effect on the impact strength in PS-PP 1/1 blends, exhibiting a nearly cocontinuous phase morphology. The strong migration of SB2 copolymers to the interface and of SB1 copolymers away from the interface were detected during the annealing of compatibilized PS-PP 4/1 blends. (C) 2004 Wiley Periodicals, Inc.