To understand the differences in conformational behavior and reactivity of oxygen- and sulfurcontaining 1,3,5-heterocyclohexanes, the enthalpies of formation and sublimation of 1,3,5-trithiane, 1, have been measured. The numerical value of the enthalpy of formation for this compound in the solid state is -8.6 ± 2.6 kJ mol-1, while the corresponding value in the gaseous state is 84.6 ± 2.6 kJ mol-1. The value for the enthalpy of sublimation is 93.2 ± 0.2 kJ mol-1. Standard ab initio molecular orbital calculations at the G2(MP2), G2, and G3 levels were performed, and the calculated enthalpies of formation are compared with the experimental data. These experimental and theoretical studies support the relevance of through-space lone pair-lone pair electronic repulsion in the sulfur heterocycle.