The synthesis of hard-core/soft-shell and soft-core/hard-shell polymers by a two-stage semi-continuous microemulsion polymerization process is reported here. In the first stage, high-solid polymer seeds (> 30 wt%) of slightly crosslinked polystyrene or poly(butyl acrylate) were obtained; then, the other monomer was added semi-continuously to form the shell. The effects on the mechanical properties (Young's modulus, ultimate properties, hardness and impact energy) of the ratio of rigid-to-soft and soft-to-rigid polymers were studied. It was found that the material becomes stiffer and presents a lower elongation at break as the amount of the rigid polymer increases. The mechanical properties also depend on the location of the hard and soft polymers. Experimental mechanical properties were compared with the predictions of the Kerner and the equivalent box models. Comparison with the predictions of the Kerner model suggests that phase inversion occurred in the case of hard-core/soft-shell materials. Phase inversion was corroborated by transmission electron microscopy. The thermodynamically preferred morphology, according to theory, is that of soft-core/hard-shell, regardless of the order of addition of monomers. Experimental data follow closely the predictions of the equivalent box model only for soft-core/hard-shell polymers. (c) 2005 Published by Elsevier Ltd.