Micron-sized particles with a narrow size distribution are prepared by ink-jet printing technology. Droplets of a polymer solution are printed with the nozzle submerged into an aqueous phase. The particles are formed upon removal of the solvent for the polymer. The particle size can be accurately predicted from the initial drop size and the polymer concentration. The method is also suitable for the preparation of monodisperse capsules with a well-defined core and shell using an additional non-solvent for the polymer. Hollow capsules are prepared by removal of the non-solvent from core of the capsules using freeze-drying. This leads to gas-filled capsules with a well-defined polymeric shell and a diameter in the range of 5 μm, which may be applied as an ultrasound contrast agent.