The enthalpy of formation of methanethial-S,S-dioxide (CH2SO2) has been determined by the use of isodesmic and nonisodesmic reactions, with individual enthalpies calculated by employing density functional methods and the CBS-QB3 model chemistry. After assessing the possible errors in the methodology, it was concluded that this value can be expressed as Delta(f)Hdegrees(298)(CH2SO2) = -144.7 +/- 8.4 kJ/mol.