The mechanism of Co-C bond photodissociation in methylcobalamin (MeCbl) and ethylcobalamin (EtCb1) has been examined by means of time-dependent density functional theory (TD-DFT). The Present contribution extends our recent study (J. Phys. Chem. B 2007, 111, 2419-2422) where relevant excited states involved in the photolysis of MeCbl have been identified. To obtain reliable structural models, the high-resolution crystal structure of MeCbl was used as the source of initial coordinates. The full MeCbl was simplified by replacing the corrin side chains by H atoms and the resulting geometry was optimized. The model of EtCb1 was generated from the simplified structure of MeCbl by replacing methyl group with ethyl. For both models, the low-lying singlet and triplet excited states have been computed along the Co-C coordinate at TD-DFT/BP86/6-31G(d) level of theory. These calculations reveal that the photodissociation process is mediated by the repulsive (3)(sigma(Co-C) -> sigma*(Co-C)) triplet state. The overall mechanism of photodissociation for both systems is similar but energetic details are different, reflecting the difference in Co-C bond strength in MeCbl and EtCbl. In both cases the key intermediate involved in Co-C bond photodissociation is identified as first excited state (SI). The S, intermediate has mixed character: it can be described as predominantly d(Co) -> pi*(corrin) metal-to-ligand charge transfer (MLCT) state with contribution from a bond to corrin charge transfer (SBLCT) where upon electronic excitation the electron density shifts from the axial N-1m-Co-C bonding to corrin ligand. The optimized geometry of the S-1 indicates that the structure of the corrin remains essentially unchanged in comparison to ground state (S-0). The major structural change occurs in the N1mCo-C moiety, which becomes bent with elongated Co-C bond in S-1 state. Finally, it is proposed that the photolysis of Co-C bond is in fine with the mechanism of heme-CO photolysis, where participation of the d(Fe) -> pi*(porphyrin) has been suggested.