Density function theory (DFT) B3LYP method was employed to study the mechanism of the reaction of CH₃SH and H₂O₂ with the 6-311＋G(d,p) basis sets. Geometries of the stationary points were completely optimized. The transition states were validated by the vibrational analysis and the internal reaction coordinate (IRC) calculations. The vibration analysis and the IRC analysis testified the authenticity of intermediates and transition states. The results indicate that the reaction can occur through three elementary steps which involve two paths. The second step IM1 to CH₃SO₂H constitutes the rate-determining step and the first path is the main path. The corresponding activation energies are 157.3 and 109.7 kJ/mol.

Key words: thiol, hydrogen peroxide, reaction mechanism, topological analysis of electronic density