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A continuously operated tubular crystallizer system with an inner diameter of 2.0 mm has been successfully operated. It allows the crystallization of active pharmaceutical ingredients (APIs) under controlled conditions. Acetylsalicylic acid (ASA) which was crystallized from ethanol (EtOH) was used as the model substance. An ethanolic suspension of ASA-seeds was fed into the tubular crystallizer system, where it was mixed with a slightly undersaturated ASA−EtOH solution that was kept at an elevated temperature in its storage vessel. Supersaturation was created via cooling and the seeds grew to form the product crystals. This work mainly focuses on the proof-of-concept and on the impact of the flow rates on the product crystals and the crystal size distribution (CSD). All other parameters including concentrations, temperatures, and loading of seeds were kept constant. Higher flow velocities generally resulted in reduced number and volume mean diameters, due to reduced tendency of agglomeration and decreased time for crystal growth due to shorter residence times of the suspension in the tube. Generally, all experiments unmistakably led to shifting of volume density distributions toward significantly larger values for product crystals in comparison to the seeds and were capable of yielding product masses in a g/min scale. [ABSTRACT FROM AUTHOR]