saturated solution of the target substance in the supercritical vehicle prior to introducing the solution into the particle formation vessel the flow rate of the second supercritical fluid should be high with respect to that of the vehicle target substance solution the apparatus shown schematically in fig2 is for use in producing particles of a target substance which is less than freely soluble in the chosen vehicle but much more soluble in the vehicle once modified with another solvent for instance a slightly polar target substance might not be highly soluble in supercritical carbon dioxide but much more soluble in a mixture of supercritical carbon dioxide and a few mole percent of a polar modifier in the fig2 system the vehicle in this example carbon dioxide is fed from source 21 through cooler 22 pump 23 and heat exchanger 24 to bring it into a supercritical state it is then fed into a mixing vessel 25 together with a solution of the target substance in an appropriate primary solvent from source 26 via pump 27 the solution and the supercritical carbon dioxide enter the mixing vessel through a two passage coaxial nozzle 28 of the type described above which allows the kinetic energy of the carbon dioxide to act to disperse the solution ensuring thorough mixing of the two fluids when they meet at the nozzle outlet this yields a solution of the target substance in the carbon dioxide and the primary solvent ie effectively a solution of the target substance in modified supercritical carbon dioxide the u201c modified u201d solution is maintained in a supercritical state by controlling the pressure and temperature inside the mixing vessel using pressure regulator.