Crystalline cob(II)alamin (vitamin B12r) and its solutions in various liquids react reversibly with 02 to yield a complex which is shown by epr spectroscopy to be mononuclear. In accord with the known shielding of the cobalt atom of the cobalamin against collision with large molecules, the mononuclear product of oxygenation does not combine with a second cob(II)alamin molecule to give a dicobalt complex. Eight hyperfine lines are found in the spectrum of methanolic solutions of oxygenated cob(II)alamin at temperatures close to the melting point. The coupling constant due to the cobalt nucleus is 12 G. At about 159°K there is a transition attributed to cessation of tumbling to give a spectrum different in over-all shape and in having an additional set of eight hyperfine lines. Superhyperfine structure due to the coordinated nitrogen atom of dimethylbenzimidazole is lost on oxygenation. Comparison with the hyperfine coupling constants of related binuclear complexes indicates that oxygenated cob(II)alamin may be regarded as superoxocobalamin, most of the unpaired spin density being concentrated at the coordinated 02 •" group. Detection of a reversibly formed mononuclear product of oxygenation establishes the first reaction step in the autoxidation of Bi2r and indicates the likelihood of transient occurrence of mononuclear superoxo complexes during autoxidation of hemoproteins. When cob(II)inamides are oxygenated the epr signals obtained resemble those from superoxocobalamin at temperatures below 159°K. The hyperfine coupling constants may be larger or smaller than for superoxocobalamin, depending upon whether the strength of bonding of the fifth ligand is less or greater than that for the nucleotide in the cobalamin.