Mitochondria produce reactive oxygen species (ROS) as by-products during oxidative phosphorylation. Since oxygen readily accepts free electrons, ROS such as O₂^- (superoxide), as well as H₂O₂ (hydrogen peroxide) and .OH (hydroxyl radical), are easily formed.

Overproduction of reactive oxygen species can lead to oxidative stress and result in damage of protein, lipids, and DNA.