Dispersion polymerization of methyl methacrylate in supercritical CO2 is studied in situ by turbidimetry at 65 degrees C from 2000 to 5000 psia for various concentrations of a poly( dimethylsiloxane) monomethacrylate (PDMS-mMA) macromonomer stabilizer. The average particle size, particle number density, and overall surface area are reported vs time during particle formation. Coagulative nucleation and controlled coagulation regions have been identified. They are governed by the amount of stabilizer available relative to the total surface area of the dispersion. Near the end of the controlled coagulation region, which can last tens of minutes, the particle number density approaches the final value. The time in this region is longer than predicted by the model proposed by Paine (Macromolecules 1990, 23, 3190) due to incomplete incorporation of stabilizer, solubility limitations of polymerized stabilizer in the continuous phase, and plasticization of the particles by CO2, which increase particle coagulation. Threshold values of pressure and stabilizer concentration are required to achieve a solvent quality and surface coverage sufficient to prevent uncontrolled coagulation during particle formation.