Magnetic iron oxide nanoparticles were prepared by in situ within a sulfonated styrene-divinylbenzene copolymer. The magnetic nanocomposite was characterized by X-ray diffraction, transmission electron microscopy and vibrating sample magnetometry. The size distribution of the nanoparticles was relatively narrow with good distribution within the copolymer. Nearly spherical nanoparticles, 7 nm in diameter, were identified as CoFe2O4 and had a well-defined crystalline structure. Magnetic measurements revealed that the magnetic nanocomposites are superparamagnetic.