Seeded suspension polymerization can be applied to obtain core-shell particles with particle diameters larger than 1000 mu m, which finds application in the rigid foam industry, for instance. However, depending on the operation conditions, the formation of secondary particles decreases drastically the efficiency of monomer incorporation in the seed particles. In the present work, the mechanism of secondary particles formation during seeded suspension polymerization was investigated using monomers (styrene, methyl acrylate and methyl methacrylate) and initiators (benzoyl peroxide and lauroyl peroxide) with different water solubilities and, in the case of the initiators, also different decomposition rates. Results showed that there was no seed polymer in the composition of the secondary particles but only pure polymer from the monomer feed, meaning that they were not formed by erosive breakage of the swelled seeds. The fraction of secondary particles increased when monomers with higher water solubility and initiators with decreasing water solubility were used. These results were consistent with the formation of secondary particles by homogeneous nucleation and monomer droplet nucleation. (C) 2008 Elsevier Ltd. All rights reserved.