PS/EPDM blends prepared by in situ-polymerization of styrene in the presence of EPDM are immiscible and show two phases. Furthermore, the dynamic mechanical behavior of injected specimens is quite different from that of noninjected blends. This is attributed to the differences in morphology before and after injection molding. The morphology of the noninjected blends consists of PS spherical domains covered by a thin layer of EPDM, whereas the injected blends show elastomeric dispersed phase morphology in a rigid matrix. SEM analysis was important to elucidate the changes in the dynamic mechanical behavior of PS/EPDM blends, but TEM analysis is more precise for morphological characterization and yielded the real average diameter of EPDM particles. Comparing the average diameters for the PS/EPDM blends obtained from SEM and TEM analyses, the diameters obtained from the SEM analysis are wider than those of TEM which is due to the solvent extraction effect oil the blend morphology. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 2280-2289, 2009