The immobilized Vitamin B-12 (Vit-B-12) within the nanoreactors of MCM-41 as molecular sieves was characterized by X-ray powder diffraction (XRD), FT-IR, chemical analysis and nitrogen adsorption-desorption. XRD and N-2 adsorption-desorption isotherms showed that the well ordered hexagonal structure of MCM-41 is presented and surface area, pore volumes and pore diameters decrease after immobilization of Vit-B-12 within nanoreactors of MCM-41. It was found that Vit-B-12/MCM-41 successfully catalyzes the oxygen transfer of tert-butylhydroperoxide (TBHP) to norbornene and trans-2-hexene-1-ol and formation of the corresponding epoxides with 90% reactivity and 100% selectivity. Moreover, cyclohexane, cyclohexene and cyclohexanol are converted to the corresponding alcohols and ketones. On the other hand, styrene undergoes oxidative degradation with the formation of benzaldehyde and benzoic acid. (c) 2005 Elsevier B.V. All rights reserved.