The electron-transfer oxidation and subsequent cobalt-carbon bond cleavage of vitamin B12 model complexes were investigated using cobaloximes, (DH)2CoIII(R)(L), where DH- = the anion of dimethylglyoxime, R = Me, Et, Ph, PhCH2, and PhCH(CH3), and L = a substituted pyridine, as coenzyme B12 model complexes and [Fe(bpy)3](PF6)3 or [Ru(bpy) 3](PF6)3 (bpy = 2,2′-bipyridine) as a one-electron oxidant. The rapid one-electron oxidation of (DH) 2CoIII(Me)(py) (py = pyridine) with the oxidant gives the corresponding Co(IV) complexes, [(DH)2CoIV(Me)(py)] +, which were well identified by the ESR spectra. The reorganization energy (λ) for the electron-transfer oxidation of (DH) 2Co(Me)(py) was determined from the ESR line broadening of [(DH) 2Co(Me)(py)]+ caused by the electron exchange with (DH)2Co(Me)(py). The λ value is applied to evaluate the rate constants of photoinduced electron transfer from (DH)2Co(Me)(py) to photosensitizers in light of the Marcus theory of electron transfer. The Co(IV)-C bond cleavage of [(DH)2Co(Me)(py)]+ is accelerated significantly by the reaction with a base. The overall activation energy for the second-order rate constants of Co(IV)-C bond cleavage of [(DH)2CoIV(Me)(py)]+ in the presence of a base is decreased by charge-transfer complex formation with a base, which leads to a negative activation energy for the Co(IV)-C cleavage when either 2-methoxypyridine or 2,6-dimethoxypyridine is used as the base.