In an attempt to understand the fast formation of diastereomerically enriched crystals in a Dutch resolution system, the nucleation and growth behaviour of such a system was investigated by in situ optical microscopy. Nucleation was observed to follow two pathways: nuclei crystal formation and ribbon formation. The shape of these ribbons (flat, helical or twisted) was dependent on the composition of the system. Hindrance during growth of the ribbons was observed to result in cleavage of the ribbons, creating new crystals. After continued growth, the ribbons develop into needles. This is most likely caused by the introduction of defects and not by a change in crystal polymorph. (C) 2004 Elsevier B.V. All rights reserved.