The thermodynamic stability and transformation behaviors of the polymorphs (A and C) and solvated crystals (BH (BPT.0.5H(2)O) and D (BPT . MeOH)) of the thiazole-derivative (BPT) were investigated. The thermodynamic stability and the transformation behavior were examined in relation with the chemical potential and the solubility of BPT in each crystal. The C form is basically the stable form and the A form is the metastable form at every temperature and methanol compositions (V-MeOH). However. the stability of BH and D forms remarkably changes with temperature and the methanol compositions. At 323 K with methanol compositions of 0.7 and 0.8 A BH and D forms transformed to the stable form, C. However, at 0.5 of V-MeOH A and D forms do not transform to the stable C form directly, but transform to the other metastable form of BH. This result may indicate the preferable nucleation and growth of BH crystals due to the specific solute-solvent interaction in comparison with C crystals. The small chemical potential difference between BH and C forms may also stabilize the BH form. The increase in the stability of D crystals and the decrease in the stability of BH with the methanol composition was related to the dissociation equilibrium of each crystal. At 3 13 K the BH form transformed to the other metastable form. D. while transforming to the stable form C. The transformation from the hydrated BH to the solvated D crystal seems to be relatively easy because of the similar configurations of carboxylic acid group in BPT. The thermodynamic stability of the solvated crystals of BH and D decreases with temperature. It is considered that the dissociation of both BH and D crystals is accelerated with increase of temperature. In the crystallization on adding water to BPT solution of methanol and water mixture, it became clear only the metastable form of A and B crystallizes and the crystallization of A form is remarkably accelerated by increasing the addition rate of water. (C) 2002 Elsevier Science B.V. All rights reserved.