In previous publications we described several methods to prepare iron oxide and Fe magnetic nanoparticles entrapped within poly(divinyl benzene) or carbon composite microspheres for various biomedical applications. However, these methods involved relatively complex synthetic processes, the size of the entrapped nanoparticles could hardly be manipulated and the obtained particles were mostly ferromagnetic. The present work describes a new simple approach to overcome these obstacles. For this purpose, core–shell polystyrene/(poly(divinyl benzene)/ferrocene) and polystyrene/(poly(divinyl benzene–styrene)/ferrocene) micrometre-sized particles of narrow size distribution were prepared by a room temperature swelling of uniform polystyrene template microspheres dispersed in an aqueous continuous phase with emulsion droplets of a swelling solvent, such as toluene containing the monomers styrene and/or divinyl benzene, and the initiators azoisobutyronitrile and ferrocene. The monomer(s) within the swollen uniform polystyrene template microspheres were then polymerized at elevated temperature. Superparamagnetic and ferromagnetic poly(divinyl benzene)–Fe₃O₄ and poly(styrene–divinyl benzene)–Fe₃O₄ composite microspheres of narrow size distribution were then formed by annealing the previous micrometre-sized composite particles containing ferrocene at 330 °C for 2 h under ambient atmosphere in a sealed cell, followed by the dissolution of the polystyrene template part. This study shows that the size of the Fe₃O₄ nanoparticles entrapped within the poly(divinyl benzene) and poly(divinyl benzene–styrene) matrices can be controlled by adjusting the [divinyl benzene]/[styrene] volume ratio. Superparamagnetic C–Fe micrometre-sized particles of narrow size distribution were formed by annealing the PDVB–Fe₃O₄ particles at 450 °C under hydrogen atmosphere for 2 h.