The Gaussian-3X (G3X) method has been used to calculate accurate values of the gas-phase enthalpies of formation of all five DNA and RNA nucleobases: thymine, adenine, cytosine, guanine, and uracil, whose experimental gas-phase study is a difficult problem because of the low vapor pressures. Only for uracil, the published experimental value agrees well with the theoretical one. However, the revision of available experimental data on enthalpies of formation for compounds in the solid phase and enthalpies of sublimation allowed us to recommend a set of self-consistent thermochemical data for thymine, adenine, and cytosine, which lead to good agreement between experimental and theoretical gas-phase enthalpies of formation. To obtain more reliable values of enthalpy of sublimation at 298.15 K, the heat capacity values of gaseous compounds were calculated by standard statistical thermodynamics formulas using molecular parameters determined from B3LYP/6-31G(2df,p) calculations. The following values are recommended based on G3X calculations and a revision of published experimental data: A(f)H(298)degrees(uracil, g, 298.15 K) = (-301.5 +/- 2.5) kJ.mol(-1), Delta(f)H(298)degrees(thymine, g, 298.15 K) = (-338.0 +/- 2.5) kJ.mol(-1), Delta(f)H(298)degrees(cytosine, g, 298.15 K) = (-69.5 +/- 3.5) kJ.mol(-1), and Delta(f)H(298)degrees(adenine, g, 298.15 K) = (225.7 +/- 3.5) kJ.mol(-1). Because of a lack of reliable experimental data for guanine, the theoretical G3X value is recommended for this compound: Delta(f)H(298)degrees(guanine, g, 298.15 K) = (16.0 +/- 5.0) kJ.mol(-1).