The heat of formation (DeltaH(f)) of sulfine, CH2=S=O, has been determined by the CBS-QB3 quantum chemical method, using 10 reactions, including the isodesmic reaction CH2=S=O + SO2 --> CH2 = S + SO3. The derived DeltaH(f) of sulfine, -30 +/- 6 kJ/mol at 298 K, lies midway between two previously calculated values: -9 +/- 14 and -52 +/- 10 kJ/mol. The CBS-QB3 derived DeltaH(f) (0 K) was very recently validated against the very accurate Weirmann-1 ' (W1 ') method [L.N. Heydorn, et al., Z. Phys. Chem. 215(2001) 141] and there is excellent agreement between the two methods, within 3 kJ/mol. Our recommended value is evaluated against experimental observables, such as the measured proton affinity of CH2=S=O and the appearance energy of CH2=S+-OH from dimethyl sulfoxide ions, CH3S(=O)CH3.+.