Superoxide is a reactive anion and free radical, formally O₂^-. It is produced most easily by burning rubidium or caesium in oxygen. It has an unpaired electron, is not particularly stable, and spontaneously decomposes into peroxide and oxygen 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 close to the spontaneous diffusion rate, the genetic inactivation or "knockout" of SOD produces deleterious phenotypes in organisms ranging from bacteria to mice (the latter species dies around 21 days after birth if the mitochondrial variant of SOD is inactivated.) The apparent discrepancy between the kinetic and biological data can be explained by the fact that the sponteanous dismutation is second order with respect to superoxide concentration while that catalysed by SOD is first order, i.e. superoxide levels can be significantly lowered by SOD.