The reaction mechanism of CH₃SO_x (x＝0, 1) with ³O_y (y＝1, 2, 3) has been studied at B3LYP/6-311G (d, p) level. Geometries of the stationary points have been optimized. Vibration analysis and intrinsic reaction coordinate (IRC) calculation at the same level have been applied to validation of the connection of the stationary points. The calculated results show that there are seven channels on the reaction surface of SO₂ formation, in which the reaction between CH₃S and O would occur the most easily. The channel in which CH₃S reacts with O₃ first to form CH₃SO, then CH₃SO reacts with O₃ to afford CH₃SO₂, and finally CH₃SO₂ decomposes to give CH₃S and SO₂ could also occur easily. These two channels should be the dominant reaction channels.

Key words: acid rain, density functional theory, reaction mechanism, topological analysis of electronic density