Photolysis of methylcobalamin (Co(III)corrin(CH3)L + H2O + O2 �> Co(III)(H2O)L + H2CO + OH-) shows a pronounced wave-length dependence. It is suggested that the reactive excited state is of the ligand-to-ligand charge transfer (LLCT) type and involves the promotion of an electron from the Co-C sigma-bond to a pi*(corrin) orbital. This LLCT transition mixes with the pipi*(corrin) transitions. Owing to this LLCT contribution, the pipi*-absorption bands are also photoactive but with reduced efficiency.