An apparatus is described for micron and submicron particles formation of a substance using the GAS process, comprising a particles formation vessel (22) and means for introducing a solution of the substance and a supercritical fluid into the particles formation vessel (22), wherein said means comprise a nozzle (27) having a central orifice (39) serving to carry a flow of solution, and a plurality of separate outer orifices (41) serving to carry a flow of pure supercritical fluid or a flow of supercritical fluid mixed with a modifier, such that the solvent is extracted from the solution by the supercritical fluid and precipitation of micron and submicron particles occurs. Also a process is described, carried out with such an apparatus.