1. The decay of oxyferroperoxidase to ferriperoxidase was studied by rapidly mixing solutions of ferroperoxidase with various amounts of oxygen and following the time-course of appearance of oxyferroperoxidase and its subsequent decay to ferriperoxidase by reaction with ferroperoxidase. 2. The scheme can be accommodated by and [Formula: see text] and occurs without detectable intermediates being observed in the millisecond time-range. 3. Analogue-computer simulation of the reaction is in agreement with the initial rate-limiting reaction being an intermolecular, not intramolecular, electron-donating process, and analysis of the data leads to quantitative values for the rate constants of the overall process. 4. The reaction of oxyferroperoxidase with ferroperoxidase is a model of a terminal oxidase, and the results are discussed in terms of the possible importance of this reaction in peroxidase function, and also in the light it throws on autoxidation of haem compounds.
