Optical resolution of DL-methionine hydrochloride (DL-met center dot HCl) by preferential crystallization was studied for the purification of L-met center dot HCl (the desired enantiomer) from supersaturated solutions of DL-met center dot HCl. The nucleation thresholds (NT) of DL-met center dot HCl affect the maximum resolution time suitable for preferential crystallization and also the percentage purity of the product crystals. Crystal growth rates of L-met center dot HCl single crystals both in supersaturated solutions of DL-met center dot HCl and in supersaturated solutions of pure L-met center dot HCl were measured in order to model the preferential crystallization more effectively. Results showed that the growth rate depends strongly on the relative supersaturation (especially from pure L-met center dot HCl solutions), that there is a wide crystal growth rate distribution in growth from both types of solution, and that the growth is faster from pure L-met center dot HCl solutions, as expected. A batch crystallizer seeded with L-met center dot HCl crystals was used to study the preferential crystallization, and to study the behavior of purity decrease of the product crystals during the crystallization process. The purity of the L-met center dot HCl product decreased to the equilibrium value over time, with almost no plateau at 100% purity (as is hoped for in preferential crystallizations). This is explainable by the very short induction times for nucleation in these solutions, and also that the L-met center dot HCl seed crystals may act as a template for the nucleation of the counter-enantiomer. (C) 2011 Elsevier B.V. All rights reserved.