A microhydration study of sulfur mustard (SM) was carried out using M06-2X, B3LYP, B3LYP-D3, and MP2 levels of theory with the 6-311++G­(2d,2p) basis set. The changes in energetics, structural parameters and vibrational wavenumbers following the addition of up to three discrete water molecules to SM were analyzed. We observed slight changes in the geometry of SM upon microhydration. The stability of hydrated clusters is due to weak C–H···O–H hydrogen bonds. The free energy change for the formation of the clusters is positive at room temperature and becomes exergonic when the temperature decreases. The infrared stretchings of C–Cl of SM and O–H of water are redshifted upon the addition of water molecules. The findings from this work add to the literature of hydrated SM and can be useful in its detection and subsequent destruction.