A soap-free emulsion polymerization method with the use of an amphoteric initiator, currently proposed by the authors for producing highly monodisperse, micrometer-sized polymer particles, was examined in the polymerization of styrene with a 2,2-azobis [N-(2-carboxy-ethyl)-2-2-methylpropionamidine] hydrate initiator and an NH4OH/NH4Cl pH buffer. The pH buffer was used to control the electric surface potential of particles to maintain a stable dispersion of particles and to prevent generation of new particles during the polymerization. Addition of monomer to the reaction system during polymerization Could enlarge the average size of polymer particles to 5.7 mum with a coefficient of variation of 1.5%, which is much less than the standard criteria of monodispersity, 10%. (C) 2004 Elsevier Inc. All rights reserved.