method for preparing a target substance in particulate form , by introducing into a particle formation vessel , through separate first and second fluid inlets respectively , a solution or suspension of the target substance in a fluid vehicle and a compressed fluid anti - solvent for the substance , and allowing the anti - solvent to extract the vehicle from the target solution / suspension so as to form particles of the target substance , wherein the target solution / suspension enters the vessel downstream of the point of entry of the anti - solvent and at a point which lies on or close to the main axis of anti - solvent flow , and wherein the anti - solvent has a sub - sonic velocity as it enters the particle formation vessel .