Precipitation of solids resulting from solution supersaturation is widely adopted to produce organic and inorganic powders. In fact, the rapid expansion of supercritical solution (RESS) is a new process of particle formation. Various morphologies and particle sizes can be thus produced: thin films, thin diameter fibers, needles or spherical products of narrow size distribution. The distinguishing features of this process are the fast attainment of the uniform conditions and of high supersaturations in the carrier fluid (supercritical carbon dioxide), which favor the formation of small particles, with narrow distribution. The expansion of a supercritical solution thus leads to loss of solvent power and hence to solute precipitation. The RESS is described for the production of fine powders of caffeine from supercritical carbon dioxide upon expansion. There is variety of the fluid solution expansion parameters. The product morphology, however, can vary considerably depending on the solution components and the operating conditions used in the process: solute concentration, preexpansion and expansion temperature and pressure of extraction have been shown to affect the product characteristics of the formed powder during the process. Optical photomicrographs of the formed particles are compared taking into account the variation of thermodynamic variables. Finally we discuss the variation of the density distribution and the particle sizes along a plate of deposition.