Horseradish oxyperoxidase [EC 1.11.1.7] is prepared by photolysis of an aerobic solution of the CO-peroxidase. Oxyperoxidase is decomposed to the ferric enzyme without appreciable intermediates. The decay of oxyperoxidase is greatly accelerated by the addition of dimethyl-p-phenylenediamine (DMPD). In the reduction of oxyperoxidase to the ferric enzyme, about one mole of DMPD is oxidized to its free radical in the presence of a limited amount of DMPD while three moles are oxidized in the presence of excess DMPD. Rate constants of the reaction between oxyperoxidase and various electron donors are measured. The stability of Compound I is greatly affected by the contamination with “endogenous donor,” but it has no significant effect on the stability of oxyperoxidase and the rates of cytochrome c peroxidation and hydrogen peroxide decomposition. Oxyperoxidase is also decomposed oxidatively in the presence of peroxidase Compounds I and II. The mechanism of oxyperoxidase autodecomposition and the abnormal stoichiometry of reductive titration of oxyperoxidase by DMPD are discussed on the basis of oxidative decomposition of oxyperoxidase.