We have prepared magnetite nanoparticles (Fe3O4_NPs) almost spherical in shape with average particle size of 10 nm and successfully encapsulated them in an envelope of polypyrrole (PPY) chains via an emulsion polymerization route using sodium dodecyl sulfate as surfactant. The resulting PPY-coated Fe3O4_NPs (Fe3O4_NPs/PPY) suspensions were stable with particles exhibiting a triangular prismatic morphology and an average diameter below 100 nm. In fact, all colloidal solutions were stable in aqueous media with typical -surface potential values of 33.9 mV (Fe3O4_NPs) and 20.0 mV (Fe3O4_NPs/PPY). Although X-ray diffraction studies revealed the presence of a magnetic phase Fe3O4, the identified diffraction peaks are consistent with the presence of a spinel structure of magnetite. A ferromagnetic behavior, such as lower coercive force (Hc = 0.065 T), was observed for all magnetic nanoparticles examined. The 1H NMR relaxation times T1 and T2 of selected Fe3O4_NPs/PPY samples were also measured and their relaxivities r1 (1.1 s1 mM1) and r2 (61.9 s1 mM1) compare favorably to those of contrast agents commercially used in human examinations. We suggest that the present results indicate that these hybrid nanocomposites are promising materials for the development of a platform of specialized contrast agents for 1H Magnetic Resonance Imaging. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013