It is known that H2O2 at pH 10, inactivates copper(II)-zinc(II)-SOD although not much information is available on what happens at the ligands coordinated to the two metal ions. We have reinvestigated the system through the electronic and 1H NMR spectra of the cobalt(II) and copper(II)-cobalt(II) derivatives. Such studies indicate that the coordinated residues are maintained although there is evidence of some flexibility of the donor groups. The coordination around copper is slightly more tetragonal. Azide binding to the copper ion does not cause the complete detachment of one of the histidines from the copper coordination sphere, as happens with the untreated enzyme.