Abstract The one-electron reduction of vitamin B1 2 and its coenzyme has been studied by γ-irradiating the frozen methanolic solutions of these compounds. A new optical absorption spectrum was obtained by γ-irradiation of aqueous methanol solution of vitamin B1 2 at 77°K; this spectrum changed by slight warming to that of B1 2 r. ESR spectra were also recorded. The ESR signal at g ≈ 2.3 for the irradiated solution at 77°K was ascribed to the vitamin B1 2 anion, in which an unpaired electron is localized in the σ★ orbital of the cobalt-ligand bond. Upon slight warming the ESR signal changed to that of B1 2 r. In contrast to the vitamin, the ESR spectrum of irradiated coenzyme B1 2 solution in C2 H3 O2 H3 + 2H2 O at 77°K showed no signal due to a paramagnetic cobalt complex but an asymmetric signal of g ≈ 2.0, which is attributable to an organic radical presumably derived from the adenosyl group. The optical spectrum indicates the formation of B1 2 s (diamagnetic) by the CoC bond cleavage. Upon slight warming the optical and ESR spectra changed to those ascribed to B1 2 r. The present study thus revealed that the one-electron reduction of vitamin B1 2 and its coenzyme B1 2 gives rise to the heterolytic cleavage of the CoC bonds by the selective attack of electrons, and the former produces B1 2 r and the latter, B1 2 s.