A single-stage polymerization technique, proposed for producing micron-sized polymer particles in aqueous media by Gu and Konno, was examined. Styrene was used for the monomer, potassium persulfate for the initiator, and cetyltrimethylammonium bromide and sodium dodecyl sulfate were used for the ionic surfactants to control the aggregation of the particles during the reaction. Particle-size distributions and monomer-to-polymer conversions were measured over a range of initiator concentration (2-8 mmol/dm(3) H2O), stirring speed (200-400 rpm), surfactant addition time and monomer concentration (1.1-2.2 mol/dm(3) H2O). Changes in the initiator concentration and stirring speed did not bring about significant effect on the particle-size distributions. On the other hand, the addition of time affected the coefficient of variation of the particle-size distribution, but it had no significant effect on the average diameter of the particles. An increase in the monomer concentration enlarged the average size without lowering the coefficient of variation of particle-size distribution. Monodisperse polystyrene particles with an average diameter of 3.3 mu m and a coefficient of variation of particle size distribution 7% could be prepared with the present technique.