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Desulfurization of flue gas by ozonation aided by the catalysis of a metallic ion was investigated experimentally. The results showed that the removal efficiency of SO 2 was only 30% at a 1:1 molar ratio of O 3 /SO 2 without the addition of a catalyst. However, the efficiency was substantially increased when a metallic ion was added to the absorption solution at the same ratio of O 3 /SO 2 . In particular, the performance was optimal using a Mn 2+ catalyst. Under the optimal condition given as the absorption liquid temperature of 20 °C, gas flow rate of 10 L/min, molar ratio of 1.0 for O 3 /SO 2 , and Mn 2+ concentration of 6.4 × 10 −4 mol/L, the removal efficiency of SO 2 was 99%. A reaction mechanism of SO 2 with ozone in the liquid-phase oxidation catalyzed by Mn 2+ was proposed. The advantage of ozonation desulfurization technology is that SO 2 is directly transformed into a valuable product, sulfuric acid, which provides favorable environmental and economic benefit.