Two concentrated emulsions in water were prepared : one from weakly polymerized butyl methacrylate (BMA) and the other one from a weakly polymerized mixture of acrylonitrile (AN) and styrene (St). Each of the concentrated emulsions also contained a small amount of a vinyl-terminated macromonomer (VTM), After the concentrated emulsions were partially polymerized, they were mixed and subjected to complete polymerization. This generated a blend of poly(butyl methacrylate) (PBMA), binary copolymer AN-co-St (AN-St), and networks containing chains of VTM and those formed from different monomers. The networks constitute compatibilizers between the PBMA and AN-St. Such a preparation method, in which the components and compatibilizer are generated simultaneously, was called self-compatibilization. The blend possesses excellent tensile properties and toughness compared with the ternary copolymer AN-St-BMA and with the solution blends of PBMA/AN-St. The generation of the compatibilizers and the compatibilization mechanism were investigated via kinetic studies. The effects of the VTM, polymerization conditions, and the weight ratio of AN/St were also examined.