Laminar peeling is often observed on the injection-molded surface of the saturated rubbermodified plastics (EVASTAN), which are prepared by copolymerizing styrene and acrylonitrile in the presence of ethylene–vinylacetate copolymer (EVA). However, the delamination rarely occurs upon higher grafting and crosslinking EVASTAN with preactivated EVA, which has pendent unsaturation. The microstructure of these EVASTAN resins was observed by electron microscopy at the end of polymerization and after processing. It was found that the delamination is caused by the agglomeration and deformation of the EVA particles and is prevented by increasing the stability of the particles by grafting and crosslinking.