Polystyrene/poly(vinyl acetate) latex nanoparticles with a core-shell morphology in an emulsifier-free emulsion polymerization system were prepared with purified styrene and vinyl acetate (VAc) as monomers and 2,2'-azo bis(2-amino propane) dihydrochloride (ABA,2HCl) as the initiator and emulsifier. The optimized conditions of polymerization of VAc, on top of the already-formed polystyrene as a core polymer, with a core-shell morphology were obtained using various parameters such as volume ratio of the first and second stages, type of process, and reaction time. The morphologic structure of the nanoparticles was studied by scanning electron microscopy and transmission electron microscopy. The latex nanoparticles and polymers were characterized by differential scanning calorimetry. (c) 2006 Wiley Periodicals, Inc.