Composite particles were prepared by seeded emulsion polymerization at 70°C using K2S2O8 as initiator and two different nonionic surfactants. Monodisperse polystyrene latex particles were used as seed and methyl methacrylate was used as second-stage monomer. When the surfactant, polyethylene oxide–propylene oxide (Pluronic F-108), was used, the final particle morphology showed that the PMMA (core) was partially covered by polystyrene. However, when nonyphenol polyethylene oxide (Igepal Co-990) was used as surfactant, one observed a reversed type of encapsulation (i.e., PS core is partially engulfed by PMMA). The interfacial tensions of the polymer phase against water containing the appropriate surfactant were measured by the drop-volume method and used in a mathematical model based on thermodynamic analysis to predict the equilibrium particle morphology. The observed particle morphologies were found to differ from the predicted morphologies at low conversion of the second-stage monomer but agreed with it at higher conversion.