An oxidative detoxification of sulfite, which originates from sulfur dioxide taken up into a leaf, has not yet been fully understood. In this study, we discuss that redox reactions between sulfite and H2O2 have an important role for the detoxification of sulfite. Sulfite was oxidized by H2O2 and during the redox reaction, oxygen consumption was observed. The oxygen consumption was partially inhibited by superoxide dismutase, indicating that O2- is generated during the redox reaction. Oxidation of sulfite by H2O2 was also observed in the presence of ascorbic acid, and during the oxidation, no significant oxidation of ascorbic acid and no consumption of oxygen were observed. Sulfite inhibited catalase of cell-free extracts of spinach, pea and broad bean leaves. These results suggest that when leaves are fumigated with SO2 in the light, catalase is inactivated resulting in the accumulation of H2O2 in leaves, which can oxidize sulfite without generating active oxygen species like O2- as long as ascorbate is present in leaves.