The mechanism for the atmospheric oxidation of DMSO has been studied. For the first time, all the possible channels in the DMSO + OH reaction are studied together theoretically, and their corresponding rate constants have been evaluated under the variational transition-state formalism. Three different channels have been characterized: an addition-elimination process to form MSIA (CH3SOOH) and CH3, a H-abstraction pathway to give CH3SOCH2, and H2O, and a nonkinetically relevant S(N)2-type reaction to form methanol and CH3SO. In agreement with previous experimental and theoretical works, the main product in the DMSO + OH reaction turns out to be the MSIA, with a branching ratio at 298.15 K around 97%. The effects of pressure in the global rate constant have also been analyzed.