Ultrafine well-dispersed Fe3O4 magnetic nanoparticles were directly prepared in aqueous solution using controlled coprecipitation method. The synthesis of Fe3O4/poly (2-acrylamido-2-methylpropane sulfonic acid) (PAMPS), Fe3O4/poly (acrylamide-co-2-acrylamido-2-methylpropane sulfonic acid) poly(AM-co-AMPS) and Fe3O4/poly (acrylic acid-co-2-acrylamido-2-methylpropane sulfonic acid) poly(AA-co-AMPS) -core/shell nanogels are reported. The nanogels were prepared via crosslinking copolymerization of 2-acrylamido-2-methylpropane sulfonic acid, acrylamide and acrylic acid monomers in the presence of Fe3O4 nanoparticles, N,N'-methylenebisacrylamide (MBA) as a crosslinker, N,N,N',N'-tetramethylethylenediamine (TEMED) and potassium peroxydisulfate (KPS) as redox initiator system. The results of FTIR and 1H-NMR spectra indicated that the compositions of the prepared nanogels are consistent with the designed structure. X-ray powder diffraction (XRD) and transmission electron microscope (TEM) measurements were used to determine the size of both magnetite and stabilized polymer coated magnetite nanoparticles. The data showed that the mean particle size of synthesized magnetite (Fe3O4) nanoparticles was about 10 nm. The diameter of the stabilized polymer coated Fe3O4 nanogels ranged from 50 to 250 nm based on polymer type. TEM micrographs proved that nanogels possess the spherical morphology before and after swelling. These nanogels exhibited pH-induced phase transition due to protonation of AMPS copolymer chains. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012