The particle sizes of the pharmaceutical substances are important for their bioavailability. The bioavailability can be improved by reducing the particle size of the drug. In this study, salicylic acid and taxol were micronized by the rapid expansion of supercritical fluids (RESS). Supercritical CO2 and CO2 + ethanol mixture were used as solvent. Experiments were carried out to investigate the effect of extraction temperature (318-333 K) and pressure (15-25 MPa), pre-expansion temperature (353-413 K), expansion chamber temperature (273-293 K), spray distance (6-13 cm), cosolvent concentration (ethanol, 1, 2, 3, v/v, %) and nozzle configuration (capillary and orifice nozzle) on the size and morphology of the precipitated salicylic acid particles. For taxol, the effects of extraction pressure (25, 30, 35 MPa) and co-solvent concentration (ethanol, 2, 5, 7, v/v, %) were investigated. The characterization of the particles was determined by scanning electron microscopy (SEM), optical microscopy, and LC-MS analysis. The particle size of the original salicylic acid particles was L/D: 171/29-34/14 mu m/mu m. Depending upon the different experimental conditions, smaller particles (L/D: 15.73/4.06 mu m/mu m) were obtained. The particle size of taxol like white crystal powders was reduced from 0.6-17 mu m to 0.3-1.7 mu m The results showed that the size of the precipitated salicylic acid and taxol particles were smaller than that of original particles and RESS parameters affect the particle size. (C) 2006 Elsevier B.V. All rights reserved.