Abstract Addition reactions of alcohols to olefins were catalyzed by hydrophobic vitamin B12 derivatives under aerobic irradiation conditions at the room temperature range. Ethyl vinyl ether, 1-pentene, and cyclohexene were adopted as olefins, while heptamethyl methylaquacobyrinate perchlorate and heptamethyl cyanoaquacobyrinate perchlorate were used as the starting complexes. Heptamethyl cobyrinate, which was generated from heptamethyl methylaquacobyrinate perchlorate by aerobic photolysis, is an appropriate catalyst for investigation of the reaction mechanism because the alkylated complex having the cobalt–carbon σ-bond, the reaction intermediate, has a long lifetime sufficient for its identification in the dark. On the other hand, heptamethyl cyanocobyrinate, which was derived from heptamethyl cyanoaquacobyrinate, is superior to heptamethyl cobyrinate as the catalyst for the addition reaction. The reaction proceeds via light-promoted decomposition of the alkylated complex, and the catalyst is regenerated upon autoxidation by air. These hydrophobic vitamin B12 derivatives are particularly effective homogeneous catalysts for the addition of an alcohol to an electron-rich olefin. Plausible reaction schemes for the catalytic cycle have been presented on the basis of experimental findings.