Density functional theory using a variety of functionals (i.e., BP86, B3LYP, B3LYP-D, B3PW91, PBE1PBE, mPW1PBE, mPW3PBE, and mPW1PW91) in combination with a polarizable continuum solvent model is applied to compute the standard reduction potentials of methylcobalamin (MeCbl) and adenosylcobalamin (AdoCbl) cofactors, respectively. A fairly good agreement between experiment and theory is obtained when the reduction potentials are computed in dimethylforamide solvent using BP86/6-31+G* level of theory. The computed reduction potentials of MeCbl and AdoCbl cofactors are predicted within 0.1–0.2 V of their experimental values. The reliability of the calibrated protocol is further testified when an acceptable degree of reproducibility (experiment vs. theory) is achieved with regard to the reduction potential of the cob(II)alamin/cob(I)alamin couple. The calibrated theoretical strategy is then exploited to understand the role of the upper axial ligand in governing the reduction potentials of alkylcobalamins. It is noted that the electron donating axial ligands tend to depress the reduction potential while electron withdrawing axial ligands (fluorinated ligands) raise the reduction potentials of the alkylcobalamins. The electronic structure calculations imply that the computed reduction potentials of alkylcobalamins are directly correlated with the energies of their lowest unoccupied molecular orbitals (ELUMO values). Thus it is concluded that the ELUMO values of alkylcobalamins that depend upon the electronic nature of the upper axial ligands serve as the key descriptors of their reduction potentials. © 2012 Wiley Periodicals, Inc.