Ab initio calculations executed at the CAS-SDCI/CASSCF/DZ(2df,2d,p)+f(S) level of theory have been employed to determine the heat of formation, Delta H-f, of the parent sulfine, CH2=S=0, 1, relative to six experimentally well-known anchors : Delta H-f(1) = -3 +/- 14 kJ/mol (0 K). The standard G2 and G2(MP2) methods yield lower values, -18 and -30 kJ/mol, respectively. It is inferred that the geometry optimization procedure in G2, which uses MP2(FULL)/6-31G*, does not accurately reproduce the experimental structure of 1. A larger basis set (MP2/6-311 + G(2df,2p) leads to the correct structure and to a Delta H-f(1) = -2 kJ/mol (0 K). Our recommended value at 298 K, Delta H-f(1) = -9 kJ/mol, represents a significant revision upwards of a previously estimated value (-50 +/- 22 kJ/mol). The proton affinity of 1 (to generate CH2SOH+) is calculated to be 787 kJ/mol (298 K).