A process for preparing an emulsion or a dispersion comprising crystalline particles of at least one active principal that comprises contacting a solution of at least one solute in a solvent or mixture of solvents in a first flowing stream with a non-solvent in a second flowing stream wherein the non-solvent and solvent are not miscible with each other, so as to form an emulsion or a dispersion. Super saturation is brought about by suitable means, and the emulsion or dispersion is subjected to ultrasonic irradiation to mediate crystal nucleation.