The mechanism of oxygen activation at a dicobalt bis-μ-hydroxo core is probed by the implementation of synthetic methods to isolate reaction intermediates. Reduction of a dicobalt(iii,iii) core ligated by the polypyridyl ligand dipyridylethane naphthyridine (DPEN) by two electrons and subsequent protonation result in the release of one water moiety to furnish a dicobalt(ii,ii) center with an open binding site. This reduced core may be independently isolated by chemical reduction. Variable-temperature 1H NMR and SQUID magnetometry reveal the reduced dicobalt(ii,ii) intermediate to consist of two low spin Co(ii) centers coupled antiferromagnetically. Binding of O2 to the open coordination site of the dicobalt(ii,ii) core results in the production of an oxygen adduct, which is proposed to be a dicobalt(iii,iii) peroxo. Electrochemical studies show that the addition of two electrons results in cleavage of the O-O bond.