Bi-Seeded emulsion polymerization was investigated in the presence of polymethyl methacrylate (PMMA) and polystyrene (PSt). The experimental results indicate, when using styrene (St) as the second-stage monomer, the latexes had bimodal size distribution and the two kinds of seeds had competition for St. It is found that when the second-stage polymerization was carried out with large PSt latex particles and small PMMA latex particles as seeds, the PMMA seeds increased more evidently and more St polymerized on them. The possible reason is that the specific surface area of seeds decides the polymerization locus and St is apt to polymerize on smaller seeds. Nevertheless, when using large PMMA and small PSt as bi-seeds, the PMMA seed particles grew more obviously and showed a higher chance of obtaining more St than that of the PSt seed particles. The explanation is the formation of PMMA/PSt composite particles has lower interfacial free energy change, which has more significant effect on the polymerization locus of second-stage monomer than the specific surface area. Adding methods have little effect on the distribution of second-stage monomer in bi-seeded polymerization system, but through dropping method, it is easier to form composite particles with core-shell structure. In addition, anomalous particles were produced when adding crosslinker into St. (C) 2004 Elsevier B.V. All rights reserved.