The effect of mixing conditions on the morphology, molten-state viscoelastic properties, and tensile impact strength of polystyrene/polyethylene (80/20) blends compatibilized with styrene-butadiene block copolymers containing various numbers and lengths of blocks was studied. Under all mixing conditions, an admixture of a styrene-butadiene block copolymer led to a finer phase structure and to an increase in the dynamic viscosity, storage modulus, and tensile impact strength. The effects were stronger for S-B diblock with a short styrene block than for S-B-S-B-S pentablock with long styrene blocks (where S represents styrene and B represents butadiene). For all blends mixed longer than 2 min, the mixing time had only a small effect on their morphology and properties. Surprisingly, the localization of S-B diblock copolymers was strongly dependent on the rate of mixing. The mixing rate had a nonnegligible effect on the viscoelastic properties of the compatibilized. blends. (C) 2003 Wiley Periodicals, Inc.