A series of latex interpenetrating polymer networks (LIPNs) were prepared via two-stage soap-free emulsion polymerization of styrene on cross-linked poly(2-acetoxyethyl methacrylate) (PAEMA) seed latexes, using potassium persulfate as initiator. It was found that a compositional gradient was present when PAEMA seeds cross-linked either lightly or highly were used. The polystyrene (PS) phase is localized near the particle center in the former case, while it is segregated near the surface in the latter case. A uniform distribution of PS phase in LIPN was formed, if moderately cross-linked PAEMA seed was used. All the LIPNs appeared to be microphase-separated, and increase of cross-linking degree in seed latexes decreased the PS-rich domain size. The results were explained by the particle growth mechanism that involved the formation of surface-active oligomeric radicals in water phase, adsorption of the radicals onto monomer-swollen particle/water interface, and chain propagation in the interface with subsequent phase migration dominated by the competitive effects of thermodynamics and kinetics.