Direct crystallization is a convenient and generally efficient method to increase the optical purity of partially resolved enantiomers. The design of chiral purification processes based on crystallization requires measurement of solubility of all stable phases, i.e. enantiomers and racemic mixtures, in the process solvent. Enantiomeric systems have been classified by the nature of the solid phases formed as racemic compounds, conglomerates or solid solutions. In rare cases, however, enantiomers form crystalline addition compounds with a stoichiometry other than I(S)A(R). These structures are generally known as "anomalous racemates". The solubility and melting point diagrams of each type of enantiomeric system exhibit characteristic features which have significant implications on the efficiency of the crystallization to enhance optical purity. In this study, we report the formation of a non-equimolar 3:1 addition compound in the RIS 3-hydroxy-4-(2,4,5-trifluorophenyl) butanoic acid. (c) 2007 Elsevier B.V. All rights reserved.