Preferential crystallization (PC) is a process to separate enantiomers. The efficiency of seeded, isothermal PC was enhanced using tailor-made additives to inhibit the crystallization of the counter-enantiomer. The inhibition of D-asparagine (D-Asn) monohydrate using D-glutamic acid (D-Glu) and D-aspartic acid (D-Asp) as additives in PC was investigated by comparing the purity, yield, and particle size distribution after PC of L-AsnH(2)O from DL-AsnH(2)O. The amount of pure L-AsnH(2)O solid product that can be produced before crystallization of the counter-enantiomer is higher when using the additives D-Asp and D-Glu. However, the crystal size of L-AsnH(2)O increases faster in PC without additives than in PC with additives. This means that the additives inhibit not only the crystallization of D-AsnH(2)O but also the crystal growth of L-AsnH(2)O.