A new polymerization system, herein referred to as a Winsor I-like system, has been successfully used for polymerization of styrene at room temperature. The system consists of a microemulsion (lower) phase which is topped off with styrene. The polymerization takes place only in the microemulsion phase where a water-soluble redox initiator ammonium persulfate/N,N,N’,N’-tetramethylethylenediamine (APS/TMEDA) is present, while the styrene phase acts only as a single monomer reservoir. Unlike microemulsion polymerization which requires higher concentrations of surfactant, this new system needs only about 1.0 wt% dodecyltrimethylammonium bromide (DTAB) for producing as high as 15 wt% polystyrene microlatex. Rather monodispersed polystyrene particles (D(w)/D(n) = 1.13) were obtained from the system that produced nanoparticles smaller than 100 nm in diameter. The factors that affect the polymerization are discussed.