A rotating-bomb combustion calorimeter specifically designed for the study of sulfur-containing compounds [J. Chem. Thermodyn. 1999, 31, 635] has been used for the determination of the enthalpy of formation of thiane sulfone, 4, ΔfH°m(g) = -394.8 ± 1.5 kJ mol-1. This value stands in stark contrast with the enthalpy of formation reported for thiane itself, ΔfH°m(g) = -63.5 ± 1.0 kJ mol-1, and gives evidence of the increased electronegativity of the sulfur atom in the sulfonyl group, which leads to significantly stronger C-SO2 bonds. Given the known enthalpy of formation of atomic oxygen in the gas phase, ΔfH°m(O,g) = +249.18 kJ mol-1, and the reported bond dissociation energy for the S=O bond in alkyl sulfones, BDE(S=O) = +470.0 kJ mol-1, it was possible to estimate the enthalpy of formation of thiane sulfoxide, 5, a hygroscopic compound not easy to use in experimental calorimetric measurements, ΔfH°m(5) = -174.0 kJ mol-1. The experimental enthalpy of formation of both 4 and 5 were closely reproduced by theoretical calculations at the G2(MP2)+ level, ΔfH°m(4) = -395.0 kJ mol-1 and ΔfH°m(5) = -178.0 kJ mol-1. Finally, calculated G2(MP2)+ values for the bond dissociation energy of the S=O bond in cyclic sulfoxide 5 and sulfone 4 are +363.7 and +466.2 kJ mol-1, respectively.