The release control of 5-chloro-2-methyl-4-isothiazolin-3-one (CMI) (biocide) by forming clathrate crystals with TEP (1,1,2,2-tetrakis(4-hydroxyphenyl)ethane) as the host was investigated. The CMI concentration released from TEP-CMI crystals increases with the amount of TEP-CMI clathrate crystals, however, it is limited. In the concentration change of TEP the peak appears and afterwards the concentration approaches to the constant value. The appearance of the peak in the TEP concentration changes is related with the crystal structure change of TEP-CMI crystals and the rate of the structural change is retarded with increase of the amount of crystals. The release mechanism of CMI is cleared to be the combination of the dissolution of TEP-CMI crystal and the crystallization of the new solid phase. The composition of the new solid phase was determined as TEP . CH3OH . HO (TEP-SOL crystal). The crystal morphology is different between the TEP-CMI crystal and TEP-SOL crystal. With the increase of water composition (V-H) from 0.5 to 0.67 and 0.91 in the solvent, CMI concentration released from TEP-CMI crystals and the solubility of TEP decreased. The rate of the structural change of TEP-CMI crystal decreased with the increase of water composition. (C) 2003 Elsevier B.V. All rights reserved.