A single stage preparation method, based on soapfree emulsion polymerization with an amphoteric initiator, is proposed for producing monodisperse, micron-sized polymer particles. The aggregation and stability of polymer particles, which have ionizable groups arising from initiator radicals, can be controlled by adjusting the PH of the reaction system. The present method was examined in the polymerizations of styrene employing an amphoteric initiator, 2,2'-azobis [N-(2-carboxyethyl)-2-2-methylpropionamidine], and a pH buffer, NH3/NH4Cl. In the polymerizations, pH was ranged from 8.1 to 8.8 and monomer concentration from 1.1 to 2.2 mol/dm(3) H 20 for an initiator concentration of 8 mmol/dm(3) H2O at 65degreesC. Selection of the reaction conditions could produce polymer particles with an average diameter of 2.5 mum and a coefficient of variation of particle size distribution as small as 1.3%.