The influence of particle size on the mechanical properties of core-shell polymers of polystyrene/poly(butyl acrylate) and poly(butyl acrylate)/ polystyrene made by a two-stage microemulsion polymerization process is reported here. In the first stage the polymerization was carried out in microemulsions containing no salt or an inorganic or organic salt to modify particle size. Smaller particles were obtained in the first stage when salts were used. The mechanical properties depend on core-shell particle size, composition and location of the parent polymers.