The invention relates to a method for producing microparticles or nanoparticles of water-soluble and water-insoluble substances by controlled precipitation, co-precipitation and self-organization processes in microjet reactors, a solvent, which contains at least one target molecule, and a nonsolvent being mixed as jets that collide with each other in a microjet reactor at defined pressures and flow rates and thereby effect very rapid precipitation, co-precipitation or a chemical reaction, during the course of which microparticles or nanoparticles are formed. In order to create such a method, with which the particle size of the resulting microparticles or nanoparticles can be specifically controlled, it is proposed within the scope of the invention that particle size be controlled by the temperature at which the solvent and nonsolvent collide, the flow rate of the solvent and the nonsolvent and/or the amount of gas, smaller particle sizes being obtained at lower temperatures, at high solvent and nonsolvent flow rates and/or in the complete absence of gas.