The functionalization of nanoparticle surfaces is required to improve the dispersion of an inorganic material inside an organic matrix. In this work, polystyrene (PS) brushes were grown on the surface of iron oxide magnetic nanoparticles with atom transfer radical polymerization and a grafting-from approach. After polymerization, the magnetic nanoparticles had a graft density of 0.9 PS chains/nm(2). A sacrificial initiator was used to obtain a satisfactory result for the control of the polymerization, as its addition had to generate a sufficient concentration of persistent radicals (deactivator). A variety of techniques, such as Fourier transform infrared spectroscopy, thermogravimetric analysis, gel permeation chromatography, water contact-angle measurements, and atomic force microscopy, were used to characterize the nanoparticles. (c) 2007 Wiley Periodicals, Inc.