Compatibilizing effects of styrene/rubber block copolymers poly(styrene-bbutadiene-b-styrene) (SBS), poly(styrene-b-ethylene-co-propylene) (SEP), and two types of poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS), which differ in their molecular weights on morphology and selected mechanical properties of immiscible polypropylene/polystyrene (PP/PS) 70/30 blend were investigated. Three different concentrations of styrene/rubber block copolymers were used (2.5, 5, and 10 wt %). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to examine the phase morphology of blends. The SEM analysis revealed that the size of the dispersed particles decreases as the content of the compatibilizer increases. Reduction of the dispersed particles sizes of blends compatibilized with SEP, SBS, and low-molecular weight SEBS agrees well with the theoretical predictions based on interaction energy densities determined by the binary interaction model of Paul and Barlow. The SEM analysis confirmed improved interfacial adhesion between matrix and dispersed phase. The TEM micrographs showed that SBS, SEP, and low-molecular weight SEBS enveloped and joined pure PS particles into complex dispersed aggregates. Bimodal particle size distribution was observed in the case of SEP and lowmolecular weight SEBS addition. Notched impact strength (a k ), elongation at yield ( y ), and Young's modulus (E) were measured as a function of weight percent of different types of styrene/rubber block copolymers. The a k and y were improved whereas E gradually decreased with increasing amount of the compatibilizer. The a k was improved significantly by the addition of SEP. It was found that the compatibilizing efficiency of block copolymer used is strongly dependent on the chemical structure of rubber block, molecular weight of block copolymer molecule, and its concentration. The SEP diblock copolymer proved to be a superior compatibilizer over SBS and SEBS triblock copolymers. Low-molecular weight SEBS appeared to be a more efficient compatibilizer in PP/PS blend than high-molecular weight SEBS.