Effects of three different linear styrene-butadiene block copolymers on morphology and selected mechanical properties of high-impact polystyrene/polypropylene blends were investigated. Scanning and transmission electron microscopy and small-angle X-ray scattering were used for the determination of the blend morphology. Impact strength, elongation at break and flexural modulus were measured for further evaluation of the compatibilizer efficiency. A similar effect on the particle dispersion was found for all three copolymers but both the styrene-terminated multiblocks influence formation of interfacial layer and consequently the related mechanical properties in a much more pronounced way than the diblock. Small-angle X-ray scattering was found to be the appropriate technique for the determination of triblock concentration necessary for covering a given interfacial area (critical concentration).