Micrometer-sized, monodisperse, magnetic composite particles were prepared by heating micrometer-sized, monodisperse, hollow polystyrene/polydivinylbenzene composite polymer particles at 200degreesC for 4 h (particles had been dipped in pentacarbonyliron) and then washed in 12 N HCl and water. The hollow polymer particles were produced by seeded polymerization by the dynamic swelling method that was proposed by authors. The magnetic composite particles contained Fe3O4, the content of which was 49% based on total weight, and were attracted easily in water by a 1650 G magnet. (C) 2003 Wiley Periodicals, Inc.