The effect of a styrene-butadiene block copolymer on the phase structure and impact strength of high-density and low-density polyethylene/high-impact polystyrene blends with various compositions was studied. For both the blends, the type of the phase structure was not affected by addition of a styrene-butadiene compatibilizer. The localization and structure of the compatibilizer in the blends were dependent on their composition. Addition of the compatibilizer improved impact strength of the blends in the whole concentration range. The improvement was the largest for blends with a low amount of the minor phase.