Heptamethyl cobyrinate perchlorate, [Cob(II)ester]ClO4, has the relevant structural features of base-off B-12r, the reduced Co(II) form of vitamin B-12. The reversible oxygenation behavior of this complex in different solvents is investigated using continuous wave (cw) EPR at X-band and compared with that of Co(EF) porphyrin complexes. Furthermore, the influence of the addition of a nitrogen base (pyridine or 1-methylimidazole) to the solutions is investigated. To determine the electronic structure of the oxygenated complexes, different pulse electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) techniques are applied. The g and cobalt hyperfine matrix and their principal axes are determined using a combination of cw-EPR at X- and Q-band. ESE (electron spin-echo)-detected EPR at W-band and Davies-ENDOR at Q-band. The experimental g and cobalt hyperfine values are found to be sensitive to the change of solvent, addition of a nitrogen base, and change in the ring structure. From the HYSCORE (hyperfine sublevel correlation) spectra measured at X-band, the interactions with the corrin nitrogen nuclei and the nitrogens of the axial base are deduced. Comparison of these data with those of oxygenated base-on Co(II) porphyrin complexes revealed shorter axial cobalt-nitrogen bonds in the Co(II) corrin than in the porphyrin case. On the other hand, the nitrogen atoms of the corrin and of the porphyrin ligands show similar, small interactions, which is due to the fact that the unpaired electron resides mainly on the O-2 fragment.