Latex particles containing equal amounts of poly (methyl methacrylate) (PMMA) and polystyrene (PS) were prepared by polymerizing styrene in the presence of PMMA seed particles using two different initiators, potassium persulfate (KPS) and tert-butyl hydroperoxide (t-BHP). Styrene was added either before polymerization (batch) or continuously during the polymerization. Particles prepared using batch addition of styrene showed no signs of a core-shell morphology, and the surface concentration of PS was lower than 50%. Particles with well-defined core-shell structures were obtained when styrene was added at a low feed rate. With t-BHP as initiator, the shell layer was compact and distinct and the surface concentration of PS high, whereas with KPS as initiator the shell layer was thicker and contained domains of PMMA. Composite particles prepared using a low styrene feed rate, but seed particles containing a chain-transfer agent, had PS domains distributed throughout the entire particle volume. Hence, the formation of particles with a core-shell structure was due to the suppression of radical transport to the interior of the seed particles, mainly because of the high internal viscosity of the latter. The structure and composition of the shell layer, however, depends on the chemical nature of the initiator.