The reactions of 3,5-di-t-butyl-o-benzosemiquinone and 9,10-phenanthrenesemiquinone with O2have been studied in the presence and absence of cobalt(II) complexes. The cobalt(II) complexes accelerate the oxygenation reactions which yield oxidative cleavage products. Based on the superior catalytic activity of cobalt Acctylacetonates relative to cobalt complexes known to bind O2, the role of the cobalt complex is proposed to be the facilitation of electron transfer from the semiquinone to the intermediate alkylperoxy radical, and not the activation of O2. 18O-labeling experiments show, the incorporation of oxygen; however, the products are a mixture of species having 0 - 4 labels incorporated. Such a pattern results from intermolecular scrambling pathways and contrasts with the clean labeling result observed in the Fe(NTA)-cataiyzed oxidative cleavage of 3,-5-di-t-butylcatechol.