Unimolecular reactions on the (CH3)(2)SOO potential energy surface have been calculated in an attempt to rationalize the reaction of singlet oxygen with dimethyl sulfide. Geometries were optimized with B3LYP/6-31+G(d) except for (CH3)2SOO which is not correctly described at that level of theory. Single-point calculations were made with a computational scheme similar to G2(MP2,SVP). A second intermediate, CH3S-(CH2)OOH, formed by intramolecular hydrogen abstraction (Delta H-a = 4.8 kcal/mol), is found to play a pivotal role in the concerted reaction pathway. From this intermediate, dimethyl sulfone can be formed in two steps. First, a hydroxyl group migrates to the sulfur (Delta H-a = 8.3 kcal/mol) followed by intramolecular hydrogen migration to the methylene group (Delta H-a = 14.4 kcal/mol). Alternatively, the CH3S(CH2)(O)OH intermediate can be protonated by trace water and deprotonated to form dimethyl sulfone, which can explain the H/D exchange process observed experimentally. A sulfurane, formed by the addition of water to (CH3)(2)SOO, is calculated to be bound by 13.5 kcal/mol.