In this work, the micronization of Cefonicid (a Cephalosporinic antibiotic) from Dimethylsulfoxide (DMSO) has been successfully performed using the supercritical antisolvent (SAS) precipitation. We obtained sub-microparticles or empty shells of Cefonicid ranging from about 0.2 mum to more than 50 mum simply varying the concentration in the liquid solution and/or the process temperature. The analysis of the morphologies observed has been supported by thermodynamic arguments; in particular, we tried to relate the different morphologies obtained to the position of the process operating point with respect to the mixture critical point (MCP) on the binary system and tried to describe the kind of vapor-liquid equilibria (VLE) modifications induced by the presence of Cefonicid. (C) 2003 Elsevier B.V. All rights reserved.