Experimental kinetics of sulfide oxidation by hydrogen peroxide presents a pH-dependent profile. In this article, it was carried out a detailed study of the mechanism and kinetics of dimethyl sulfide (DMS) oxidation by H 2 O 2 in neutral, acid, and basic aqueous medium using ab initio calculations. The results point out that DMS oxidation in neutral aqueous medium occurs through its direct reaction with H 2 O 2 . In acid medium, cluster-continuum model calculations shows that H 3 O 1 ðH 2 O 2 Þ ðH 2 OÞ 2 cluster is the best representation of the very reactive H 3 O 2 1 species. In basic medium, there is formation of the HO 2 2 species. However, the pathway involving this species has high free energy barrier, making this pathway unfeasible. The theoretical pH-rate profile is in good agreement with the experimental observations.