Polystyrene microlatexes have been prepared by conventional emulsion polymerization with a novel amphiphilic water-soluble ABA triblock copolymer, poly[2-(di-methylamino)ethyl methacrylate](15)-b-poly(propylene oxide)(36)-b-poly[2-(dimethyl-amino)ethyl methacrylate](15) (PDMAEMA(15)-PPO36-PDMAEMA(15)), as a polycationic emulsifier under acidic or neutral conditions. The ABA triblock copolymer was developed by oxyanion-initiated polymerization in our laboratory. In this study, it acted well both as a polycationic polymeric surfactant to form block copolymeric micelles for emulsion polymerization and as a stabilizer to be anchored into the polystyrene microlatex or adsorbed onto its surface. The results obtained with various copolymer concentrations and different pH media showed that microlatex diameters decreased remarkably with increased concentration of this ABA triblock copolymeric emulsifier, but were not as much affected by the pH of media within the experimental range of 3.4-7.0. The observed difference of the particle sizes from transmission electron microscopy and dynamic light scattering measurements is discussed in terms of the effect of the absorbed surfactants and their electrical double layers. This difference has led to the formation of a cationic polyelectrolyte fringe on the surface of microspheres. The final microlatexes were characterized with respect to total conversion, particle diameter, and particle size distribution as well as colloidal stability.