Mesoporous silica material was prepared to immobilize chloroperoxidase (CPO) from Caldariomyces fumago in this work. The mesoporous material (FDU-12-140) with 15-18 nm pore entrances was found to be a very suitable support not only because it allowed the entry of only a few CPO molecules (diameter around 6.2 nm) so as to avoid the aggregation of enzyme molecules that may result in a decline in its apparent activity, but also because it provided enough flip space for the enzyme during its catalytic action. Enzymatic oxidative decolorization of Crocein Orange G was employed to evaluate the catalytic performance of CPO. The decolorization efficiency of dye by immobilized CPO reached 90% within 70 min. Moreover, it displayed improved thermostability and maintained higher activity in organic solvents compared to that of free enzyme. When incubated at 80 degrees C for 1 h, 4296 of the activity of immobilized enzyme was reserved compared with that at 20 degrees C, whereas free CPO maintained only 8.5%. Even in the most hydrophilic N,N-dimethylformamide (log p = -1.0), the decolorization efficiency remained 30% by immobilized CPO, but no decolorization was observed by free CPO at the same conditions. The relative activity of immobilized CPO in the 20th reuse maintained 61% of that in the first run in decolorization of the dye.