Density functional and coupled-cluster calculations using Pople's basis sets up to 6-311++G(3df,2pd) have been employed to determine the heat of formation of sulfine, CH2SO, 1, using the isodesmic reaction CH2S + SO2 = CH2SO + SO, Other reactions, employed previously to determine the enthalpy of formation of sulfine at the CAS-SDCI/ CASSCF ab initio level, were used as well. The analysis of the results shows that: (a) the errors in the calculation of the enthalpies for the individual molecules do cancel reasonably only for the isodesmic reaction, and not for those used previously; (b) density functional methods produce smaller errors than CCSDT in the calculation of the enthalpies of formation of the molecules involved in this reaction; (c) the actual heat of formation of sulfine is determined as Delta H-f(298.15)o(l) = -52 +/- 10 kJ/mol, more in agreement with the prediction of Benson than with the ab initio value derived by Ruttink et al,; (d) the proton affinity of sulfine, calculated at the density functional level (792.0 kJ/mol) agrees reasonably well with the experimental result, 787.6 +/-2.6 kJ/mol, but the enthalpy of formation of 1 derived from this proton affinity using the assumptions of Ruttink or Bouchoux is in disagreement with the value determined previously.