Highly crosslinked poly(styrene-co-divinylbenzene) microspheres with a 1.0-2.5-mu m diameter were prepared by precipitation polymerization in neat acetonitrile. The effects of various polymerization parameters such as the monomer and initiator concentration, comonomer composition, and cosolvents on the properties of the resulting particles were studied. The particle diameter increased with the monomer concentration, whereas the particle size distribution remained almost constant. The 2,2'-azobisisobutyronitrile (based on the total monomer) was used as the initiator. The styrene fraction in the monomer composition and toluene fraction in the solvent mixture also significantly affected the morphology and size of the resulting particles. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 3144-3149, 2009