The highly monodisperse polystyrene (PS) microspheres were produced by dispersion polymerization using an amphoteric initiator, 2,2'-azobis [N'-(2-carboxyethyl)-2-2-methylpropionamidine] (VA-057). The polymerization characteristics were investigated and compared with conventional initiators. 2.2-azobis(isobutyronitrile) (AIBN) and benzoyl peroxide (BPO). The rnonodisperse PS microspheres having the coefficient of variation (C-v) of diameter all less than 4% are obtained at very low stabilizer, poly(vinyl pyrrolidone) (PVP) concentrations of 1 and 2 wt%. It is found that the size dependence of the VA-057 system, D-n proportional to [VA-057](0.267), is less sensitive than a conventional initiator system. When the same amount, 2 wt%. of AIBN, BPO, and VA-057 is used under the identical PVP concentration of 2 wt%, the D-n/C-v's are 1.95/11.57, 1.47/22.44 and 2.08 mu m/2.50%. respectively. The uniformity of particles was characterized employing an optical analyzer, Turbiscan. For the VA-057 system, the back scattering intensity is linearly reduced with time having a constant sedimentation rate of 48.98 mu m/min throughout the settling process, The uniformity of PS particles in the VA-057 system sterns from (1) the higher rate of polymerization in early stage of polymerization, followed by significantly faster reduction of the rate, and (2) good dispersion stability of primary particles. Therefore, it is found that the use of an amphoteric initiator. VA-057, is promising for producing monodisperse particles in dispersion polymerization. (c) 2006 Elsevier Inc. All rights reserved.