Ultrafast time-resolved spectroscopy was used to study the photochemistry of hydroxocobalamin (HOCbl) and aquocobalamin (H(2)OCbl(+)) in solution. Spectroscopic measurements and TD-DFT simulations provide a consistent picture of the spectroscopy and photochemistry. Excitation of H(2)OCbl(+) results in formation of an excited state followed by rapid internal conversion to the ground state (0.35 +/- 0.15 ps) through an S-1/S-0 seam at a slightly elongated Co-O bond length and a significantly elongated Co-N-Im bond length. In contrast, the initial elongation of the axial bonds in HOCbl is followed by contraction to an excited state minimum with bonds slightly shorter than those in the ground state. Internal conversion to the ground state follows on a picosecond time scale (5.3 +/- 0.4 ps). For both compounds, photodissociation forming cob(II)alamin and hydroxyl radicals (similar to 1.5% yield) requires excitation to highly excited states. Dissociation is mediated by competition between internal conversion to the S-1 surface and prompt bond cleavage.