A mechanism of Co-C bond photolysis in the base-off form of the methylcobalamin cofactor (MeCbl) and the influence of it axial base on Co-C bond photodissociation ha been investigated by tithe-dependent density functional theory (TD-DFT). At low pH, the MeCbl cofactor adopts the base off form in which the axial nitrogenous ligand is replaced by a water molecule. Ultrafast excited-state dynamics and photolysis studies have revealed that a, new. channel for rapid nonradiative decay in base-off MeCbl is opened, which,,competes with bond dissociation. To,explain thee experimental findings, the corresponding potential energy surface of the Si state was constructed as a function of Co-C and Co-O bond; distances, and the manifold of low-lying triplets was plotted as a function of Co-C bond length. In contrast to the base-on form of MeCbl in which two possible photodissociation pathways were identified on the basis of whether the Co-C bond (path A) or axial Co-N bond (path B) elongates, first, only path B is active in base-off MeCbl. Specifically, path A is inactive because the energy barrier associated with direct dissociation of the methyl ligand is higher than the barrier of intersection between two different electronic states: a metal-to-ligand charge transfer state (MLCT), and a ligand field state (LF). along the Co-O coordinate of the Si PES. Path B initially involves displacement of the water molecule, followed by the formation of an LF-type intermediate, which possesses a very shallow energy minimum With respect to the Co-C coordinate. This LF-type intermediate on path B may result in either S-1/S-0 internal conversion or singlet radical pair generation. In addition, intersystem crossing (ISC) resulting in generation of a triplet radical pair is also feasible.