Ternary elastomer blends of acrylonitrile-butadiene rubber (NBR), chlorinated polyethylene (CM), and ethylene-propylene rubber (EP) have been investigated using transmission electron microscopy (TEM). Especially the effect of comonomer content, chlorine in CM and acrylonitrile in NBR, on blend morphology is studied. The blend ratio of NBR/CM/EP is fixed at 10/20/70 by weight: EP acts as matrix and NBR and CM comprise dispersed phases. TEM observation is possible without staining due to a natural contrast between NBR and CM. From TEM micrographs it is clear that NBR/CM/EP blends show encapsulated structures. Which component will be the encapsulating layer depends on comonomer contents. For blends containing high acrylonitrile content (NBR46), CM forms the encapsulating layer; whereas, for blends with low acrylonitrile (NBR16), NBR16 encapsulates CM. The encapsulation behavior of NBR/CM/EP blends is interpreted in terms of a spreading coefficient concept combined with solubility parameter and melt viscosity differences between NBR and CM. Our calculation based on simple thermodynamic considerations explains the morphology observed in this study except for the NBR16/CM29/EP blend.