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Composite materials consisting of polystyrene-coated iron nanoparticles were prepared by the thermal decomposition of iron pentacarbonyl in the presence of polystyrene-tetraethylenepentamine dispersants. The nanocomposites contain both simple core-shell particles of 10-20 nm diameter, and more complex particles (20-100 nm) made from the agglomeration of several core-shell particles. Electron diffraction revealed that the core was composed of iron, in contrast to the iron nitride (Fe 3 N) reported for similar conditions with a polyisobutylene-based dispersant. The materials exhibit hysteresis and ranges from 7.6 to 29.3 emu/g. The coercivity goes through a maximum for particles of 25 nm diameter. Removal of unbound dispersant from the materials greatly increases the magnetization.