PS/AES blends were prepared by in situ polymerization of styrene in the presence of AES elastomer, a grafting copolymer of poly(styrene-co-acrylonitrile) - SAN and poly(ethylene-co-propylene-co-diene)-EPDM chains. These blends are immiscible and present complex phase behavior. Selective extraction of the blends' components showed that some fraction of the material is crosslinked and a grafting of PS onto AES is possible. The morphology of the noninjected blends consists of spherical I'S domains covered by a thin layer of AES. After injection molding, the blends show morphology of disperse elastomeric phase morphology in a rigid matrix. Two factors could contribute to the change of morphology: (1) the stationary polymerization conditions did not allow the mixture to reach the equilibrium morphology,- (2) the grafting degree between I'S and AES was not high enough to ensure the morphological stability against changes during processing in the melting state. The drastic change of EPDM morphology from continuous to disperse phase has as consequence a decrease in the intensity of the loss modulus peaks corresponding to the EPDM glass transition. However, the storage modulus at temperatures between the glass transition of EPDM and PS/SAN phases does not change significantly. This effect was attributed to the presence of the SAN rigid chains in the AES. (C) 2009 Wiley Periodicals, Inc. Journal of Applied Polymer Science 113: 2638-2648,2009