Polystyrene-silica core-shell composite particles are prepared by one-step emulsion polymerization with a nonionic initiator VA-086, solely stabilized by silica nanoparticles. The silica nanoparticles are successfully incorporated as the shell, likely due to the fact that the nanoparticles are thermodynamically favorable to adsorb and remain at the liquid-liquid interfaces during the emulsion polymerization. The silica content, determined by thermogravimetric analysis, is approximately 20 wt% in the composite particles. In addition, we further explore the polymerization mechanism by studying the particle growth as a function of initiator concentration and reaction time: when the initiator/monomer ratio is increased from 0.83 to 2.5 wt%, the particle size at 24 h reaction time decreases for a fixed monomer amount. possibly due to a larger number of nuclei at the initial stage of polymerization. Further increasing the initiator/monomer ratio to 4.2 wt% does not continually decrease the particle size, which may be limited by the stabilization provided by a fixed concentration of silica nanoparticles. The surface coverage also changes with initiator-concentration and reaction time although the underlying mechanism is not fully understood. (C) 2009 Elsevier Inc. All rights reserved.