The supercritical antisolvent (SAS) process was used to modify the solid-state properties of sulfathiazole and chlorpropamide. Acetone, methanol, and ethyl acetate were employed as solvents for the pharmaceutical compounds, and carbon dioxide was used as an antisolvent. The effects of process parameters on the precipitate crystals such as carbon dioxide injection rate, type of solvent, and temperature were investigated. The SAS processed crystals show more ordered appearances with clean surfaces and sharp angles compared with the unprocessed particles. The crystal habit changed from tabular to acicular when the carbon dioxide injection rate increased. The X-ray diffraction patterns of the two compounds revealed variations of crystallinity and crystal orientations depending upon the injection rate, where the degree of crystallinity was found to be inversely proportional to the rate of injection. The analysis of differential scanning calorimetry indicated that both the injection rate and temperature influence the crystal's thermal stability which is related to the solid-solid transition and fusion. The crystal size significantly increased when the nucleation and crystal growth took place at a slow rate. (C) 2002 Elsevier Science B.V. All rights reserved.