Polystyrene/AES blends formed by the in situ polymerization of styrene in the presence of poly (acrylonitrile-g-(ethylene-co-propylene-co-diene)-g-styrene) (AES) were prepared. AES is a commercial elastomer obtained by the radical copolymerization of styrene and acrylonitrile in the presence of an ethylene-propylene-diene terpolymer (EPDM). The polystyrene/AES blends presented two phases: an EPDM elastomeric phase dispersed in a rigid matrix. The phase behavior was strongly affected by the polymerization temperature. The blends showed higher thermal stability than the polystyrene homopolymer because of the stabilizing effect of EPDM incorporation. The mechanical properties were influenced by the polymerization temperature and blend composition. The blend prepared at 60 degrees C with 13.0 wt % AES presented an enhancement of 60% in the impact resistance, whereas the blend prepared at 80 degrees C with 21.8 wt % AES presented an enhancement of 150% in the strain at break. Both blends had these properties improved with a small loss in the Young's modulus. (C) 2007 Wiley Periodicals, Inc.