The nucleation and transformation of two polymorphs of glycine (alpha-and gamma-forms) have been investigated using sodium chloride (NaCl) as additive. The two polymorphs were unambiguously examined by using Powder XRD, FTIR, TGA and DSC. The polymorphic transition temperature was determined by both DSC and solubility measurements. The nucleation behavior of the two forms shows that the polymorph of initial nuclei strongly depends on the NaCl concentration and initial concentration of glycine. In aqueous solution without additive or at low concentration of NaCl, the alpha-form spontaneously nucleates, while at high concentration of NaCl and low initial concentration of glycine the gamma-form. crystallizes preferentially. Based on the above observations, it is proposed that the presence of NaCl could affect the packing arrangements of glycine molecules in aqueous solution through coulombic interactions, leading to the competitive nucleation of the two polymorphs. Furthermore, the solution-mediated polymorphic transformation experiments prove that the nucleation of gamma-form could be significantly promoted in the presence of NaCl. This appears to be the first detailed experimental investigation of the occurrence domain as well as the polymorphic transformation of metastable form alpha and stable form gamma of glycine crystallized from NaCl aqueous solution. (C) 2007 Elsevier B.V. All rights reserved.