The oxidation of native horseradish peroxidase to Compound II by photochemically generated inorganic free radicals [Formula: see text], [Formula: see text], and [Formula: see text] was observed. The rates of Compound II accumulation for different rates of free radical generation and different reagent concentrations have been measured. The interpretation of the experimental data allowed us to estimate the rates of the reactions [Formula: see text], [Formula: see text], and [Formula: see text]: (3–6) × 106 M−1 s−1, (2.2–3.0) × 106 M−1, and [Formula: see text] respectively. Also obtained for all three oxidants were the ratios of the number of free radicals responsible for the heme oxidation to the total number of them attacking the enzyme.