The standard molar enthalpy of formation DELTA(f)H(m)-degrees{(CH3)2SO, 1} at T = 298.15 K of (CH3)2SO, dimethylsulfoxide, in the liquid state has been determined to be -(203.4 +/- 1.4) kJ . mol-1 by means of oxygen rotating-bomb combustion calorimetry. Combination of this value with the literature value of the molar enthalpy of vaporization gave -(105.5 +/- 1.5) kJ . mol-1 for DELTA(f)H(m)-degrees{(CH3)2SO, g}. The present value agrees well with a literature value based on the solution-reaction calorimetric determination of the standard molar enthalpy change for the reaction : (CH3)2S(1) + H2O2(aq) = (CH3)2SO(1) + H2O(1), by Douglas (J. Am. Chem. Soc. 1946, 68, 1072).