Polymeric particles with core-shell morphology present remarkable advantages over conventional ones. The final properties of these particles depend not only on the composition of each polymer phase, but also on its morphology, opening the possibility for tailor-made properties for each application. In this work, core-shell particles were produced on seeded suspension polymerization by using polystyrene (PS) as polymer core, or seed, and methyl metacrylate (MMA) as the shell forming monomer. The main purpose of this study was to improve the PS particles chemical resistance by coating them with a poly(methyl metacrylate) (PMMA) shell. Two synthesis routes were evaluated by varying the PS seed conversion level before the addition of MMA. The conversion level of the seeds used was 98% (Reaction 1) and 92% (Reaction 2), analyzed by gas chromatography (GC). The particles morphology was analyzed by transmission electron microscopy (TEM) and ruthenium tetroxide vapor was used for staining them to distinguish the phases. The chemical resistance was tested by particles dissolution in cyclohexane, as a selective solvent of PS, and the results were compared to pure PS. The TEM analysis showed that the core-shell morphology consists of PMMA clusters dispersed in the PS matrix and that the size and concentration of these clusters, along the radius, was highly influenced by the degree of seed conversion. The chemical resistance analysis showed that the reaction 2 particles presented the best result. This result is clearly justified by their morphology, which contains more PMMA clusters densely grouped at the surface.