AbstractMonodisperse microscopic poly (methyl methacrylate)/polystyrene composite particles of 1–10 μm diameter were prepared by seeded emulsion polymerization of styrene in poly (methyl methacrylate) seed particles which were initially prepared by dispersion polymerization. The resulting composite particles were characterized by transmission and scanning electron microscopy. The unique morphology of the composite particles comprised three types of polystyrene domains embedded in a continuous poly (methyl methacrylate) matrix: the dispersed “internal” domains in the interior, the interconnected “subsurface” domains that from a crust beneath the surface, and the separated “surface” domains at the surface. In addition to thermodynamics and kinetics, the phase separation in composite particles is affected by polymerization mechanism. The complicated particle morphology found is attributed to the coexistence of bulk polymerization and emulsion polymerization (radical absorption and desorption) mechanisms. An explanation of the formation of the observed morphology is proposed. With varying poly (methyl methacrylate)/styrene ratio, the internal viscosity and the competition between the two polymerization mechanisms resulted in a series of interesting morphologies.