Superoxide is a reactive anion and free radical, formally O2-. It is produced most easily by burning Rubidium or Caesium.It has an unpaired electron, is not particularly stable, and spontaneously decomposes into peroxide over time. In phagocytes, superoxide is produced in large quantities by the enzyme NADPH oxidase for use in oxygen-dependent killing mechanisms. Superoxide has also been implicated in the mechanisms of aging and the peroxidation of lipids.

Superoxide is scavenged by the enzyme superoxide dismutase, or SOD. Although the rate of reaction of SOD is so close to the spontaneous disassociation rate that some have questioned the biological need for SOD, the absence of SOD can lead to a form of anemia in humans.