The enthalpies of Combust. and sublimation of 1,3-dithiacyclohexane 1-oxide (1,3-dithiane sulfoxide, 2) were measured by a rotating-bomb Combust. calorimeter and the Knudsen effusion technique, and the gas-phase enthalpy of Form. was determined, ΔfH°m(g) = -98.0 ± 1.9 kJ mol-1. This value is not as large (negative) as could have been expected from comparison with thermochemical data available for the thiane/thiane oxide Ref. system. High-level ab initio Molec. orbital calculations at the MP2(FULL)/6-31G(3df,2p) level were performed, and the optimized Molec. and Electron. Struct. of 2 afforded valuable information on (1) the relative conformational energies of 2-axial and 2-equatorial-the latter being 7.1 kJ mol-1 more stable than 2-axial, (2) the possible involvement of nS → σ*C-S(O) hyperconjugation in 2-equatorial, (3) the lack of computational evidence for σS-C → σ*S-O stereoelectronic interaction in 2-equatorial, and (4) the relevance of a repulsive electrostatic interaction between sulfur atoms in 1,3-dithiane sulfoxide, which apparently counterbalances any nS → σ*C-S(O) stabilizing hyperconjugative interaction and accounts for the lower than expected enthalpy of formation for sulfoxide 2.