Carbon dioxide or ethane was used as an antisolvent medium from which organic solids dissolved in a liquid carrier solvent are precipitated and collected as finely divided solids. Solid compounds of pharmaceutical interest, which are virtually insoluble in the supercritical fluids per se, are efficiently obtained by injecting a solution of the solid into a volume of stirred supercritical fluid or compressed liquefied gas. A free-flowing powder comprising very small particles (<5 mu m) is obtained. Both continous-flow and batch operations were developed although the continuous-flow operation is preferred. Process variables including temperature, pressure, stirring rare, injection concentration, and the rate and temperature of the carrier solution injection were studied to see how they affect the physical properties of the resultant product. Recovery (yield), particle size and uniformity, bulk powder density, microscopic appearance, and surface area were measured. Carbon dioxide and ethane antisolvents, several liquid-phase injection solvents, and different solid compounds were studied. A simple unsteady-state mathematical model of this CSTR precipitator is also presented.