In this study, the mechanical properties of in situ polymerized PS/EPDM blends with different composition were evaluated before and after accelerated photoaging and compared with the properties of commercial HIPS submitted to similar aging conditions (ASTM G53). The mechanical properties of the PS/EPDM blends as well as their photochemical stability are influenced by the polymerization temperature and blend composition. Although the initial mechanical properties of HIPS are superior in comparison with the in situ polymerized PS/EPDM blends, a pronounced drop of them was observed already for short time exposure. For example, after the aging period, all PS/EPDM blends showed higher strain at break than HIPS. Because PS/EPDM blends present higher resistance to photoaging stability than HIPS, the mechanical properties of the HIPS become worse than the other blends as the aging time increases.