Abstract Time-dependent density functional theory (TD-DFT) has been applied to investigate the relevant excited states involved in the photodissociation of hydroxocobalamin (HOCbl). The TD-DFT/BP86/6-31G(d) and TD-DFT/BP86/6-311G(d,p)//BP86-631G(d) calculations suggest that the photoscission of Co–OH bond is mediated by the repulsive 1(n + dCo → σ∗Co–OH) singlet state that drops in energy along the reaction stretch coordinate and yields the products of photodissociation i.e., Cob(II)alamin and hydroxyl radical. The channel involving the repulsive triplet state may also be a possible mechanistic pathway. The present calculations thus may help in enhancing our understanding about the photophysical behavior of HOCbl which has implications in the investigation of biological complexes such as DNA and RNA.