Cu,Zn-Superoxide Dismutase (SOD) is an antioxidant enzyme that dismutates superoxide radical anions to hydrogen peroxide and oxygen molecules. In addition, SOD also exhibits peroxidase activity in the presence of H₂O₂, leading to self-inactivation and formation of potent oxidants. The peroxidase activity has been extensively studied in recent years due to its potential relationship with mutated SODs associated to familial amyotrophic lateral sclerosis, a fatal motor neuron disease. The aim of the present work is to identify DNA damage induced by the system SOD/H₂O₂. Exposure of plasmid DNA to SOD in the presence of H₂O₂ caused DNA single-strand breaks, revealed by agarose gel electrophoresis. Calf thymus DNA damage was also evaluated by the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) and 1,N²-etheno-2´-deoxyguanosine (1,N²-edGuo), quantified by HPLC coupled to eletrochemical detection and mass spectrometry, respectively. Increased levels of 8-oxodGuo and 1,N²-edGuo were observed in DNA incubated with SOD in the presence of H₂O₂. The addition of DTPA, a metal chelator, caused a drastic decrease in the level of all these lesions, suggesting the participation of copper ions released from SOD active site. In the presence of bicarbonate, the levels of 8-oxodGuo and 1,N²-edGuo in DNA were dramatically increased with SOD/H₂O₂ compared to incubations performed in the absence of HCO₃^-. Our results confirm the ability of SOD to promote biomolecule damage in the presence of H₂O₂ and bicarbonate, and point to the involvement of free copper ions and diffusible carbonate radicals in the formation of the lesions. These data contribute towards an understanding of the peroxidase activity of SOD.