The invention relates to an improved process for the production of powders of an inhalable medicament by crystallization from a supersaturated fluid containing said medicament, the method comprising passing along a tubular reactor (a) a segmented flow of that fluid comprised of discrete volumes; or (b) a fluid mixture being separated by discrete volumes of a separating fluid which is substantially immiscible with said fluid,characterized in that the crystallization is initiated by application of ultrasound