Monodisperse micron-sized polystyrene particles were produced by seeded polymerization, and the effect of seed crosslinking on styrene monomer swelling and Anal particle morphology were considered. The monomer swelling procedure using crosslinked seed particles was predicted with a thermodynamic equation. Employing this equation, the crosslinking density of the seed particles could be determined from the rate of transport of the monomer molecules to the crosslinked seed particles. From a thermodynamic viewpoint, it was elucidated that the monomer swelling procedure was strongly dependent on the seed crosslinking density. Therefore, for an effective swelling of the monomer, an optimum seed crosslinking density was required. After polymerization of these swollen particles, highly monodisperse polystyrene particles could be obtained with a size range of over 10 mu m.