Femtosecond transient absorption spectroscopy has been used to investigate the photolysis of coenzyme B12 (5‘-deoxyadenosylcobalamin). Transient kinetic measurements obtained at wavelengths between 400 and 633 nm were analyzed globally to obtain time constants. Transient spectral data obtained for time delays between 5 ps and 9 ns were analyzed by matrix decomposition to identify distinct spectral components present in the data. Photoexcitation results in homolysis of the carbon−cobalt bond forming a singlet radical pair on a picosecond time scale. The subsequent spectral changes probe conformational relaxation and geminate recombination. Analysis of the spectral data suggests that 76 ± 4% of the geminate radical pairs recombine, resulting in a quantum yield of 0.24 ± 0.04 for the formation of solvent separated radicals, in good agreement with literature values of 0.20 ± 0.03 and 0.23 ± 0.04 [Chen, E.; Chance, M. R. Biochemistry 1993, 32, 1480−1487]. The geminate recombination of the adenosyl radical with...