We report the synthesis and structural characterization of a dicobalt(III) complex with a mu-OH,mu-O-2 core, namely mu-OH,mu-O-2-[Co(enN4)](2)(X)(3) [1(ClO4)(3) and 1(BF4)(3)]. The dinuclear core is cross-linked by two N4 Schiff base ligands that span each cobalt center. The formally Co-III-Co-III dimer is formed spontaneously upon exposure of the mononuclear Co(II) complex to air and exhibits a nu(O-O) value at 882 cm(-1) that shifts to 833 cm(-1) upon substitution with O-18(2). The CV of 1(BF4)(3) exhibits a reversible {Co-III-Co-III}<->{Co-III-Co-IV} redox process, and we have investigated the oxidized {Co-III-Co-IV} species by EPR spectroscopy (g = 2.02, 2.06; S = 1/2 signal) and DFT calculations.