Monodisperse crosslinked polystyrene microspheres were prepared by the dispersion copolymerization of styrene and divinylbenzene in a mixed solvent of ethanol and H2O. 2,2'-Azobisisobutyronitrile and poly(N-vinyl pyrrolidone) were used as the initiator and steric stabilizer, respectively. The crosslinker-postaddition method was adopted through a slow addition of a crosslinking agent into the dispersion system at a certain time after the beginning of the polymerization. The effects of the postaddition recipe, postaddition beginning time, postaddition velocity, and agitation rate on the particle size, size distribution, and morphology were discussed. The resulting polymer microspheres were characterized with scanning electron microscopy and laser particle analysis. Crosslinked polystyrene microspheres with a narrow size distribution and a 12.0%, crosslinker level were obtained with a size of 1.0 mu m through the crosslinker-postaddition method. (C) 2007 Wiley Periodicals, Inc.