Monodisperse magnetic Fe3O4/poly(styrene-co-acrylamide) [Fe3O4/poly(St-AAm)] nanoparticles with diameters of 50-300 nm were prepared by emulsion polymerization with microwave irradiation in the presence of a ferrofluid coated with oleic acid (OA) and sodium dodecyl sulfate (SDS). The influence of some polymerization parameters were examined in detail, such as monomer content and the ferrofluid content. The structure of the obtained magnetic nanoparticles was characterized by Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD). The morphology and size of the magnetic nanoparticles were determined by transmission electron microscopy (TEM) and photon correlation spectroscopy (PCS). The weight values of iron oxide incorporated into the magnetic nanoparticles were calculated by thermogravimetric analysis (TG). The saturation of the magnetic nanoparticles was characterized by a vibrating-sample magnetometer (VSM). TEM and XRD revealed that the Fe3O4 nanoparticles were incorporated into the shells of poly(St-AAm). TG provided the weight percent of iron oxide, and the maximum value observed was 45.9%. The magnetic nanoparticles were easily separated in a magnetic field. (c) 2007 Elsevier Ltd. All rights reserved.