Monodisperse poly(chloromethylstyrene-co-divinylbenzene-80) microspheres of 4-6-mu m diameter were prepared by precipitation copolymerization in neat acetonitrile and in acetonitrile/toluene mixtures. These particles have clean surfaces due to the absence of any added stabilizer and up to 0.5 cm(3)/g pore volume, depending on the comonomer ratio and on the amount of toluene cosolvent. The effects of comonomer and cosolvent ratios on microsphere formation and morphology are described.