[3] The superoxide anion, •O−2, and the hydroperoxyl radical exist in equilibrium in aqueous solution: The pKa of HO2 is 4.88.

Unlike •O−2, which has reducing properties, HO•2 can act as an oxidant in a number of biologically important reactions, such as the abstraction of hydrogen atoms from tocopherol and polyunstaturated fatty acids in the lipid bilayer.

Gaseous hydroperoxyl is involved in reaction cycles that destroy stratospheric ozone.

It is also present in the troposphere, where it is essentially a byproduct of the oxidation of carbon monoxide and of hydrocarbons by the hydroxyl radical.

As hydroperoxyl is quite reactive, it acts as a "cleanser" of the atmosphere by degrading certain organic pollutants.