The toughness of high-impact polystyrene based on gamma-radiation-vulcanized natural rubber (NR) was effectively enhanced by the addition of a block copolymer at the NR/polystyrene interface. An NR/polychloroprene (CR) composite latex particle was produced by the heterocoagulation technique for its use as a model in a compatibilization study. A nonionic surfactant (Tween 80), whose molecule bore poly(ethylene oxide) (PEO), was adsorbed onto the CR particle and allowed to form a complex between PEO and an indigenous surfactant (protein-lipid) on the NR particle surface. The presence of CR-Tween on the outer layer of the obtained composite polymer particle was characterized by the particle size, zeta potential, and glass-transition temperature. That the film cast from the heterocoagulated latex showed better oil resistance than the NR film confirmed the NR/CR core-shell structure.