A variety of organic substances used in pharmaceutical and agro-alimentary industries are often required as fine powders with a controlled size. The RESS process (rapid expansion of supercritical solution) allows the micronization of thermally labile materials and the formation of small particles with a similar morphology and a narrow size distribution. The goal of this study is to improve the description of the jet/particle system in the RESS process. An experimental investigation shows that the particle structure of processed caffeine could be enormously altered when recrystallization chamber conditions were varied. Indeed, long needles, yarns, or fibers of the material could be obtained by changing the operating pressure and temperature. Numerical simulations were performed to describe the effects of hydrodynamic parameters upon the developed jet and the recrystallized particles. These calculations showed isovalues and profiles of velocity, density, and temperature of the carbon dioxide jet by which recrystallized particles were transported and deposited on the flat plate.