The Haber–Weiss reaction generates •OH (hydroxyl radicals) from HO (hydrogen peroxide) and superoxide (•O) catalyzed by iron ions. It was first proposed by Fritz Haber and his student Joseph Joshua Weiss in 1932.
This reaction has long been studied and revived in different contexts, including organic chemistry, free radicals, radiochemistry, and water radiolysis. In the 1970, with the emerging interest for the effect of free radicals onto the ageing mechanisms of living cells due to oxygen (O), it was proposed that the Haber–Weiss reaction was a source of radicals responsible for cellular oxidative stress. However, this hypothesis was later disproved by several research works. The oxidative stress toxicity is not caused by the Haber–Weiss reaction as a whole, but by the Fenton reaction, which is one specific part of it.
The reaction is kinetically slow, but is catalyzed by dissolved iron ions. The first step of the catalytic cycle involves the reduction of the ferric (Fe) ion into the ferrous (Fe) ion:
:Fe  +  •O   →   Fe  +  O
The second step is the Fenton reaction: 
:Fe  +  HO   →   Fe  +  OH  +  •OH
Net reaction:
:•O + HO   →   •OH + OH + O