A swelling and penetration process is proposed to prepare magnetic polymer microspheres. Micron-size polystyrene (PS) particles were swollen in an aqueous solution of N-methyl-2-pyrrolidone (NMP) and then mixed with superparamagnetic iron oxide nanoparticles. The magnetic nanoparticles were able to diffuse into polymer microspheres and were entrapped within the polymer microspheres. The saturation magnetization of resultant magnetic polymer microspheres increased as increasing magnetic nanoparticle concentrations were added to the swelling mixture. A higher ratio of NMP-to-water led to a greater swelling but a larger loss of polymer mass due to polymer chain dissolution in the NMP solution. Sodium dodecyl sulfate (SDS) in the NMP aqueous solution significantly enhanced the swelling and penetration process. The use of SDS could not only shorten the process time but also lower the required NMP concentration. The proposed method also worked well in preparing magnetic polymers microspheres with other styrene-based copolymer beads like poly(styrene-glycidyl methacrylate) (PS-GMA). (C) 2008 Elsevier Ltd. All rights reserved.