A method for crystallizing a substance, especially an inhalable medicament, comprising the following steps: (a) dissolving said substance in a solvent, to produce a solution of said substance, (b) producing a segmented stream which consists of alternating segments of said solution of the substance and of a transport medium, (c) introducing said segmented stream into the first end of a retention stretch, which retention stretch has a first end and a second end, so that said segmented stream passes from said first end to said second end of said retention stretch, and exits from the second end of the retention stretch, (d) cooling the retention stretch, to thereby bring about crystallization of the substance in the segments of solution, in the segmented stream, as said segmented stream passes through the retention stretch, and (e) separating the crystals of material from the segmented stream as it exits from the second end of the retention stretch. Also disclosed is an apparatus for carrying out the method.