Porous poly(2-hydroxyethyl methacrylate-coethylene chmethacrylate) [p(HENIA-co-EDNIA)] microspheres have important applications in chromatography. In this study, p(HENIA-co-EDNIA) microspheres with controllable size were successfully prepared by modified membrane emulsification technique. The CV value of the obtained n-Licrospheres was as low as 14.4%. An oil phase mainly composed of toluene was permeated through a membrane with uniform pores into an aqueous phase to form fairly uniform seed emulsion. The secondary emulsion composed of HEMA and EDMA was prepared by a homogenizer. Then, the two emulsions were mixed together. HEMA and EDMA diffused into the aqueous phase, and then were absorbed by the seed droplets. The swollen droplets were further polymerized to prepare monodisperse porous microspheres. It was found that HEMA/EDMA ratio and toluene/monomer ratio showed a significant influence on swelling degree, diameter, and morphology of the obtained microspheres. The swelling degree of the microspheres increased with the increase of toluene/monomer ratio, whereas the average diameter increased with decrease of the ratio. Microspheres with controllable size (28-73 mu m) and porosity were obtained, by varying HEMA/EDMA ratios and toluene/monomer ratios. It can be expected that the p(HEMA-co-EDMA) microspheres with controllable size and porosity will provide many advantages in their applications. (c) 2007 Wiley Periodicals, Inc.