The effect of styrene-butadiene block copolymers (SB) witli varying number of blocks and length of styrene blocks on the morphology, rheology, and impact strength of 4/1 polystyrene/ low-density polyethylene (PS/ LDPE) blends was studied. The scanning and transmission electron microscopy and X-ray scattering were used for determination of the size of LDPE particles and the localization and structure of SB copolymers in blends. It is shown that the dependence of the LDPE particle size on the amount of added SB and localization of SB copolymers in blends is predominantly controlled by the length of their Styrene blocks. It follows from thermodynamic considerations that the reason is the difference in composition asymmetry between SB with short and long styrene blocks. Coalescence of particles of SB having short styrene blocks at the surface of LDPE droplets and movement of SB with long styrene blocks to the PS-LDPE interface were observed during annealing of PS/LDPE/SB blends. Pronounced migration of SB copolymer during annealing shows that their localizations in blends in steady state on long steady mixing and at thermodvnamic equilibrium are different. The values of tensile impact strength of PS/LDPE/SB blends correlate well with the size of LDPE particles and the amount of SB at the interface. Viscosity of BS/LDPE/SB depends on molecular structure of SB copolymers by a manner different from that of tensile impact strength. The results of this Study and literature data lead to the conclusion that the compatibilization efficiency of SB copolymers for a certain polystyrenepolyolefin pair is a function of not only molecular parameters of SB but also of the polystyrene/polyolefin ratio, the amount of SB in a blend, and mixing and processing conditions. (c) 2006 Wiley Periodicals, Inc.