Ultrafast transient absorption spectroscopy has been used to investigate the primary photochemistry of methylcobalamin. Approximately 27% of the initially excited methylcobalamin undergoes a bond homolysis on a subpicosecond time scale. The remaining 73% forms a metastable photoproduct with a spectrum similar to that of cob(III)alamin compounds. The ultraviolet absorption spectrum of the metastable photoproduct exhibits a prominent gamma-band at 340 nm, characteristic of a cob(III)alamin with a very weak axial ligand. The metastable photoproduct recovers to the ground electronic state of methylcobalamin on a 1.2 +/- 0.5 ns time scale, leaving only cob(II)alamin (and presumably methyl radical) at 9 ns. The primary photochemical yield of cob(II)alamin is determined largely by the branching ratio between the two photoproduct channels. A 40 ps transient absorption difference spectrum of methylcobalamin bound to methionine synthase indicates that the branching ratio and initial production of cob(II)alamin is not changed in the enzyme-bound cofactor. The substantial photolysis protection afforded by the enzyme must be attributed to structural and electronic effects which enhance the intrinsic rate of recombination of the radical pair, rather than to suppression of primary bond homolysis.