This study investigates the formation of sub-micrometric particles of PMMA-co-BMA via a compressed CO2 assisted-atomization process using acetone as a solvent and compressed carbon dioxide as a spraying medium. The experimental results show that lower concentrations of the polymer solution, higher temperatures in the saturator, and optimized volume flow ratios can effectively reduce the mean particle size. The phase diagram of the binary system can qualitatively represent the optimized volume flow ratios, in which the composition of CO2 in the feed streams was near the bubble points at saturator temperatures. The precipitation kinetic parameters were determined by the particle size distributions with the aid of the MSMPR population balance equation. The effect of several factors such as the concentration of polymer solution, volume flow ratio, and saturator temperature on the mass-weighted mean size of micronized polymer particles can be reasonably correlated. This study found primary nucleation to be dominant in the compressed CO2 assisted-atomization process, and diffusion may govern particle growth. (C) 2012 Elsevier B.V. All rights reserved.