The atmospheric oxidation mechanisms of reduced sulfur compounds are of great importance in the biogeochemical sulfur cycle. The CH₃S radical represents an important intermediate in these oxidation processes. Under atmospheric conditions, CH₃S will predominantly react with O₂to form the peroxy radical CH₃SOO. The formed CH₃SOO has two competing unimolecular reaction pathways: isomerization to CH₃SO₂, which further decomposes into CH₃and SO₂, or a hydrogen shift followed by HO₂loss, leading to CH₂S. Previous theoretical calculations have suggested that CH₂S formation should be the dominant pathway, in disagreement with existing experimental results. Our large active space multireference configuration interaction calculations agree with the experimental results that the formation of CH₃and SO₂is the dominant route and the formation of CH₂S and HO₂can, at most, be a minor pathway. We support the calculations with new experiments starting from the OH + CH₃SH reaction for CH₃S formation under low NO_xconditions and find a SO₂yield of 0.86 ± 0.18 within our reaction time of 7.9 s. Model simulations of our experiments show that the SO₂yield converges to 0.98. This combined theoretical and experimental study thus furthers the understanding of the general oxidation mechanisms of sulfur compounds in the atmosphere.