Vitamin B(12)-TiO(2) heterogeneous hybrid catalyst (B(12)-TiO(2)) mediated ring-expansion reactions and their reaction mechanism were clarified by various spectroscopic methods. Diffuse reflectance UV/VIS (DR-UV/VIS) studies showed that B(12)-TiO(2) was activated to form the supernucleophilic Co(I) species of B(12) by irradiation with ultraviolet light. DR-UV/VIS and electron spin resonance (ESR) studies then elucidated the formation of the photo-sensitive intermediate on heterogeneous surfaces. The photo-excited B(12)-TiO(2) catalyzed ring-expansion reactions were efficient and eco-friendly. This catalytic efficiency was strongly dependent on the kind of solvent. The oxidation reaction of the solvent by holes at TiO(2) nanoparticle surfaces had a crucial role in the overall catalytic reaction.