One of the key processes of SEDS precipitation is droplet formation due to jet break-up at the exit of an injection device. In this work, a prefilming atomizer was designed on the basis of the mechanisms of atomization and applied to the SEDS process to precipitate ephedrine from ethanol solution using supercritical CO2 as antisolvent with the aim of evaluating the efficiency of the atomizer and studying the influence of operating variables (concentration, pressure, temperature, solution flow rate, and CO2 flow rate) on sizes of the particles micronized. The solution to be atomized was driven through a coaxial annular passage with spiral slots in the atomizer as a thin film swirling with 45. The atomizing CO2 was driven through another passage (the inner capillary) to form a jet stream to impinge on the film at 45. Upon violent interaction with jet streams, the solution sheet was effectively disintegrated into fine drops and the mixing of supercritical fluid (SF) and solution was intensified for increased transfer rates. Long needle-like or short rod-like uncoagulated particles were obtained by the SEDS process in a wide range of experimental conditions. The mechanisms that control particle sizes were explained in terms of liquid atomizing mechanism, nucleation, and growth processes of particles. Particle size did not seem to depend on pressure and temperature in all the experiments performed. (C) 2004 Elsevier B.V. All rights reserved.