Biochemical decomposition of catechol and 4-chlorocatechol, degradation intermediate products of 4-chlorophenol, was investigated using enzymes and their immobilized forms by clay minerals. An oxygenase that can initiate oxidative ring-fission of aromatic compounds was obtained via cloning of its gene (cphA-1) encoding hydroxyquinol dioxygenase contained in Arthrobacter chlorophenolicus A6 and overexpression and purification of the enzyme. The enzyme expressed in vitro was then immobilized onto the clay mineral (montmorillonite). Michaelis-Menten kinetic analysis was conducted to compare the expressed enzymes and the immobilized biocatalysts with regard to their degradation activity and capability to sustain activities under severe environments. Vmax values for the immobilized enzymes were smaller than those for the originally cloned enzymes, indicating that loss of enzyme activity was accompanied in the enzyme immobilization process. However, the immobilized enzyme demonstrated far more stable degrading activity in response to wide environmental changes such as marked variation of pH, temperature, and ionic strength. This supported that the enzyme immobilization can provide great advantages for its field application and also should be useful for establishing the concept of ecological green U-City.