In the crystallization of molecular complex (co-crystal, clathrate complex), polymorphism in regard to the host structure frequently appears. Previously, we studied the release process of the biocide, CMI (5-chloro-2-methyl-4-isothiazolin-3-one) from the molecular complex with TEP (1,1,2,2-tetrakis(4-hydroxyphenyl)ethane) (TEP center dot 2CMI) in methanol-water mixed solvents. It was clear that the release process of the biocide (CMI) is composed of the transformation from the TEP center dot 2CMI crystal to a more stable molecular complex crystal with solvent. In this work, the crystallization was performed in the methanol solutions including TEP and CMI at constant temperature (298 K and 308 K). It appeared that two kinds of TEP molecular complexes (TEP center dot 2CMI and TEP center dot 2MeOH) crystallize competitively. The crystallization zone of each molecular complex was shown in the map using the coordinates of initial concentrations of TEP and CMI. In the boundary zone both molecular complexes appeared and the transformation from TEP center dot 2CMI to TEP.2MeOH was observed, indicating that the stable form is TEP center dot 2MeOH. Without the boundary zone the corresponding stable form crystallized in each zone. The value of the initial concentration ratio of CMI/TEP for the selective crystallization of TEP center dot 2CMI was higher at 298 K (1.54) than that (1.36) at 308 K. The equilibrium concentrations of TEP and CMI in the presence of two molecular complexes were expressed using the dissociation constants of the molecular complexes and it was indicated that the dissociation of TEP center dot 2CMI highly increases with temperature (C) 2012 Elsevier B.V. All rights reserved.