Zinc-substituted horseradish peroxidase is oxidized by K2IrCl6 to a characteristic state which retains one oxidizing equivalent more than the zinc peroxidase. The oxidized enzyme gives an optical absorption spectrum similar to that of compound I of peroxidase and catalase, and a g = 2 electron paramagnetic resonance signal which has an intensity corresponding to the porphyrin content. It is reduced back to the zinc peroxidase by a stoichiometric amount of ferrocyanide or by a large excess of K3IrCl6. From the equilibrium data, the value of E0' for the zinc peroxidase couple is estimated to be 0.74 V at pH 6. The oxidized zinc peroxidase is also formed by the addition of H2O2 or upon illumination with white light. The rate constants for the oxidation by K2IrCl6 and H2O2 at pH 8.0 are 8 x 10(5) and 8 x 10(2) M-1 s-1, respectively. No essential spectral change can be observed when K2IrCl6 is added to the metal-free peroxidase (protoporphyrin--apoperoxidase complex) or to zinc-substituted sperm whale myoglobin.