The potential energy profile associated with CH3 and OH losses from the dimethyl sulfoxide radical cation, CH3SOCH3.+, 1, has been examined at the G2(MP2,SVP) level. Isomerization of 1 into its aci-tautomer, CH3S(OH)CH2.+, 2, by a 1,3-hydrogen migration constitutes the initial and energy-determining step of both dissociations. This explains the observation of identical appearance energies for the corresponding fragment ions. Heats of formation values of 702, 794, and 795 kJ/mol are obtained from atomization energies at the G2(MP2,SVP) level for CH2SOH+, CH3SO+, and CH2SCH3+, respectively, The kinetics of the reactions 2 --> CH2SCH3+ + (OH)-O-. and 2 --> CH2SOH+ + (CH3)-C-. have been examined by using a RRKM-type orbiting transition state theory. Explicit consideration of the rotational effect is crucial, inducing the latter process to be dominant at a high internal energy of the precursor ions 2. This offers the reason for why the m/z 63 (CH2SOH+) ions are more abundant than the m/z 61 (CH2SCH3+) in the mass spectrum of dimethyl sulfoxide even though the OH loss represents the less energy demanding reaction.