In chemistry, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (O₂). Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen.

The reduction of molecular oxygen (O₂) produces superoxide (^•O−2), which is the precursor to most other reactive oxygen species:

${\displaystyle {\ce {O2{}+e^{-}->\ ^{\bullet }O2-}}}$

Dismutation of superoxide produces hydrogen peroxide (H₂O₂):

${\displaystyle {\ce {2H+{}+\ ^{\bullet }O2^{-}{}+\ ^{\bullet }O2^{-}->H2O2{}+O2}}}$

Hydrogen peroxide in turn may be partially reduced, thus forming hydroxide ions and hydroxyl radicals (^•OH), or fully reduced to water:

${\displaystyle {\ce {H2O2{}+e^{-}->HO^{-}{}+\ ^{\bullet }OH}}}$

${\displaystyle {\ce {2 H+ + 2 e- + H2O2 -> 2 H2O}}}$

In a biological context, ROS are byproducts of the normal metabolism of oxygen. ROS have roles in cell signaling and homeostasis. ROS are intrinsic to cellular functioning, and are present at low and stationary levels in normal cells. In plants, ROS are involved in metabolic processes related to photoprotection and tolerance to various types of stress. However, ROS can cause irreversible damage to DNA as they oxidize and modify some cellular components and prevent them from performing their original functions. This suggests that ROS has a dual role; whether they will act as harmful, protective or signaling factors depends on the balance between ROS production and disposal at the right time and place. In other words, oxygen toxicity can arise both from uncontrolled production and from the inefficient elimination of ROS by the antioxidant system. During times of environmental stress (e.g., UV or heat exposure), ROS levels can increase dramatically. This may result in significant damage to cell structures. Cumulatively, this is known as oxidative stress. The production of ROS is strongly influenced by stress factor responses in plants, these factors that increase ROS production include drought, salinity, chilling, defense of pathogens, nutrient deficiency, metal toxicity and UV-B radiation. ROS are also generated by exogenous sources such as ionizing radiation generating irreversible effects in the development of tissues in both animals and plants.