Emulsifier-free miniemulsion polymerization of styrene was investigated in the presence of potassium persulfate (KPS) as an anionic initiator and cetyl alcohol as a costabilizer using ultrasonic irradiation and comparison of this procedure with conventional emulsifier-free emulsion polymerization showed that this method has a remarkably higher polymerization rate (Rp), smaller size of particles, and narrower molecular weight distribution via gravimetric measurement, transmission electron microscopy (TEM), and gel permeation chromatography techniques, respectively. Then, the encapsulation of magnetite (Fe3O4) and titanium dioxide (TiO2) nartoparticles with polystyrene was investigated using this procedure. Attempt to encapsulate magnetite nanoparticles failed; however, the encapsulation of titanium dioxide nanoparticles was successfully carried out via this procedure using KPS in both cases. TEM proved the presence of TiO2 nanoparticles in polymer particles, and thermogravimetric analysis was used to determine the percentage of TiO2 in the products. (c) 2007 Wiley Periodicals, Inc.