The geometries of CH3S(OH)CH3 (DMSOH) and DMSOH + O-2 have been investigated using ab initio and density functional theories. Reaction paths for the interaction between DMSOH and O-2 are established and potential energy surfaces/paths are calculated. On the basis of this information theoretical models for calculating the rate constants for the reactions DMSOH + O-2 --> DMSO + HO2 (where DMSO is used as abbreviation for CH3S(O)CH3) and DMSOH + O-2 + M --> DMS(OH)(OO) + M (where DMS(OH)(OO) is used as abbreviation for CH3S(OH)(OO)CH3) have been developed and rate constants for these reactions have been estimated. For these two reactions the rate constants at 298 K are estimated to be 1.74 x 10(-12) cm(3) molecule(-1) s(-1) and 8.63 x 10(-34) cm(6) molecule(-1) s(-1), respectively. After adjustment of the termolecular rate constants to typical atmospheric conditions, the atmospheric implications of DMSOH reaction with O-2 are discussed. The main conclusions from this work are: the most dominant channel for the outcome of the reaction DMSOH + O-2 is DMSO + HO2. The reaction CH3SOH + CH3O2 does not occur. On the other hand, this study shows that contributions to the total rate from the DMS(OH)(OO) reaction channel is possible. The sensitivity studies performed indicate that the DMS(OH)(00) channel provides contribution to at most 50%. This is the first theoretical calculation of the rate constants involved in the DMSOH reactions with molecular oxygen.