The relevant excited states involved in the photolysis of methylcobalamin (MeCbl) have been examined by means of time-dependent density functional theory (TD-DFT). The low-lying singlet and triplet excited states have been calculated along the Co-C bond at the TD-DFT/BP86/6-31g(d) level of theory in order to investigate the dissociation process of MeCbl. These calculations have shown that the photodissociation is mediated by the repulsive (3)(sigma(Co-C) -> sigma*(Co-C)) triplet state. The key metastable photoproduct involved in Co-C bond photolysis was identified as an S-1 state having predominantly d(Co) -> pi*(corrin) metal-ligand charge transfer (MLCT) character.