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Peroxidases catalyze the dehydrogenation by hydrogen peroxide (H₂O₂) of various phenolic and endiolic substrates in a peroxidatic reaction cycle. In addition, these enzymes exhibit an oxidase activity mediating the reduction of O₂ to superoxide (O₂^·−) and H₂O₂ by substrates such as NADH or dihydroxyfumarate. Here we show that horseradish peroxidase can also catalyze a third type of reaction that results in the production of hydroxyl radicals (^˙OH) from H₂O₂ in the presence of O₂^·−. We provide evidence that to mediate this reaction, the ferric form of horseradish peroxidase must be converted by O₂^·− into the perferryl form (Compound III), in which the haem iron can assume the ferrous state. It is concluded that the ferric/perferryl peroxidase couple constitutes an effective biochemical catalyst for the production of ^˙OH from O₂^·− and H₂O₂ (iron‐catalyzed Haber–Weiss reaction). This reaction can be measured …