Poly(methyl methacrylate-co-styrene) composite latices were prepared by thermally initiated seed emulsion (co)polymerization of styrene (ST), methyl methacrylate (MMA), or ST and MMA employing a PST or PMMA seed in the absence of conventional initiators. The changes of particle morphology, observed by transmission electron microscopy (TEM), were investigated by varying seed particle component, the weight ratio of monomer to seed polymer, monomer composition, and employing pre-swelling of the seed particles. The size distribution of polymer particles obtained from thermally initiated emulsion (co)polymerization was improved by employing the seed process. Hemisphere-like, sandwich-like, core-shell, and inverted core-shell particle morphologies were observed depending upon the polymerization conditions. The pre-swelling of seed particles did not affect the morphology of final particles. The particle morphologies, obtained from the thermal process, were compared with those obtained from conventional seed emulsion polymerization. The incorporation of an initiator fragment SO4- to polymer chain ends seemed to allow the PST chains to gain some hydrophilicity. From the observation of particle morphology, the hydrophilicity of involved polymers were in the following order: PMMA with ionic (-SO4-) chain ends > PMMA with no ionic ends > PST with ionic ends > 60% MMA P(MMA-co-ST) with no polar ends > 40% MALA P(MMA-co-ST) with no polar ends > PST with no polar ends.