The dispersion of ethylenepropylenediene terpolymer (EPDM) rubber in a polystyrene (PS) matrix is an alternative for improving the weathering resistance of high-impact polystyrene (HIPS), which commonly contains polybutadiene as an impact modifier. However, EPDM and PS are immiscible, and compatibilizers are required to improve the final properties. In this study, EPDMPS blends were prepared by melt mixing, and the compatibilizing effects of two block copolymers on these blends was studied. The materials were analyzed by scanning electron microscopy, tensile and impact tests, and exposure to UV light for 20 days. The addition of styrenebutadienestyrene block copolymer (SBS) and styreneethylene-co-butylenestyrene block copolymer (SEBS) improved the dispersion of EPDM in the PS matrix. The compatibilized blends showed lower tensile properties, but an important increase in the impact strength was observed compared to the noncompatibilized blend. With regard to the impact strength, SBS was more effective than SEBS as a compatibilizer. All of the blends showed higher UV resistances than the commercial HIPS, and the best performance was observed in the blends compatibilized with SEBS. These results are discussed and explained in terms of the blends' morphologies and chemical compositions. (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012