The O2 −‐generating step of plant peroxidases during their catalytic cycle is represented by the decay of compound III (CoIII) into ferriperoxidase, which most likely involves the dissociation of a ferric‐O2 − complex to yield the ferric form of the enzyme and O2 −. Diphenylene iodonium chloride (DPI), at 50–100 μM, does not significantly enhance the stability of CoIII of peroxidase, as judged by the values of k decay, and therefore, DPI appears to have no effect on the O2 −‐generating step of plant peroxidases. From these results, it is concluded that caution should be exercized when considering peroxidase as a possible enzyme target of O2 −‐mediated plant physiological processes sensitive to DPI inhibition.