An experimental study of hydrodynamics role in the supercritical antisolvent (SAS) process is here presented. jet dispersion and atomization of a liquid phase into a dense one have been characterized for miscible systems and for commonly used conditions in SAS processes. The critical atomization jet velocity tends to the same value for all the systems studied. The dispersion has also been studied in the case of an organic solution containing the solute to be precipitated. The presence of sulfathiazole in a concentration up to 10 wt% does not influence the liquid jet dispersion. The nucleation zone has been located: crystals are not formed in the plain jet but in the entities resulting from the jet break-up. The observation of the crystals formed shows that the key-point is the mixing state more than the atomization state to form fine particles. However, atomization conditions lead to more homogeneous powder characteristics. (C) 2009 Elsevier B.V. All rights reserved.