Photoinitiated precipitation polymerization of acrylic acid (AA) and methoxy polyethylene glycol acrylate (MPEGA) was carried out in liquid carbon dioxide using ethanol as cosolvent and trimethylol propane triacrylate (TMPTA) as crosslinker. Monodisperse crosslinked microspheres were obtained with size polydispersity index as low as 1.027. The particle yield rose sharply after irradiation and reached to 52% at 5 min, then gently increased to 65% at 60 min, suggesting that most of microspheres were produced in the first 5 min of irradiation. The particle number increased incessantly till very late stage of reaction, which could be attributed to the direct generation of stable nuclei from the fast crosslinking polymerization instead of oligomer aggregation. The effect of ethanol content, feeding ratio of monomers and crosslinker concentration on the morphologies, size, and dispersity of microspheres has been investigated. X-ray photoelectron spectroscopy showed that the microspheres obtained by this procedure were covered with abundant carbon double bonds. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 4660-4667, 2011