A hybrid emulsion polymerization was formulated for synthesizing Fe3O4/ polystyrene composite latex. This system, containing binary droplets that are magnetic (Mag)-droplets with a diameter of 100-200 nm and styrene (St)-droplets with a diameter of 3-4 mu m, was obtained by mixing Mag-miniemulsion and St-macroemulsion. With extremely low surfactants concentration (<< critical micelle concentration, CMC), the nucleated loci are selectively controlled in the Mag-droplets, as the result of smaller droplet size and larger surface ratio. Both water-soluble potassium persulfate (KPS) and oil-soluble 2,2'-azobis(2-isobutyronitrile) was adopted to initiate the polymerization. In the presence of KPS, magnetic polystyrene latices with particles size of 60-200 nm, narrow size distribution, and high magnetite content (86 wt % measured by TGA) were attained. successfully. The synthesized magnetic Fe3O4/polystyrene latices assembled into well-ordered hexagonal structure in the surface of a carbon supported copper grid. The influence of various parameters on various aspects of the as-synthesized Fe3O4/polystyrene was investigated in detail: type of initiator on composite morphology, feed ratio of Mag-miniemulsion and St-macroemulsion on magnetite content, and hydrophobic agent or amount of surfactant on size and size distribution. (c) 2007 Wiley Periodicals, Inc.