The RESS process (rapid expansion of supercritical solutions) is an innovative and promising technology to produce small particles with narrow particle size distribution and it offers interesting applications for heat-sensitive organic compounds such as certain pharmaceuticals. RESS experiments were carried out with an apparatus suitable for temperatures up to 600 K and pressures up to 60 MPa. Until now, carbon dioxide has been used as a supercritical solvent and naphthalene, cholesterol and benzoic acid as solutes. These experiments led to particle sizes in the range of 1.5 to 3 mu m for naphthalene, between 0.8 and 1.2 mu m for benzoic acid, and always less than 0.35 mu m in the case of cholesterol. The diameter and number concentration of the particles are measured in our apparatus in situ and online with a 3-wavelength-extinction measurement technique, in contrast to the usual offline examination techniques reported in the literature. Besides the experimental investigations, the RESS process is modelled numerically, considering the three parts as capillary inlet - capillary -freejet. The one-dimensional time-independent flow model for the pure solvent includes heat-exchange in the capillary and the freejet, friction in the capillary and isentropic flow in the capillary inlet area. The resulting pressure and temperature changes along the expansion path are used to calculate the solute solubility in the solvent and the supersaturation of the real mixture with the Peng-Robinson equation of state. The results of these calculations are expansion rates, P, of 10(7) is, cooling rates, T, of 10(9) K/s and theoretical supersaturations of about 10(5) to 10(8) at residence times of less than 10(-6) seconds in the supersonic freejet. Moreover, in the present paper, some important aspects of the RESS process - phase behaviour of the non-ideal, dilute supercritical mixture, application of the classical nucleation theory and the mechanism of particle growth - will be analysed.