The gas phase enthalpy of formation of the amino acid containing selenium, selenocysteine, has been calculated using G3 and G4 calculations, the values being (-347.7 and -345.2) kJ . mol (1), respectively. The enthalpies of formation of the counterpart amino acids containing oxygen and sulfur, serine and cysteine, have also been calculated. The enthalpies of formation of compounds H(2)Z, C(2)H(5)ZH and (C2H5)(2)Z (Z=O, S, Se) have been calculated and compared with their available experimental values. The agreement between calculated and experimental values gives us confidence in our calculated values for the gas phase enthalpy of formation of selenocysteine. (C) 2013 Elsevier Ltd. All rights reserved.