The standard molar enthalpies of formation of urea, glycine, and L-alanine in the gaseous phase at 298.15 K were calculated by the high-level Gaussian-3X method. The agreement with the available experimental data is very good for urea and glycine and, thus, supports the high accuracy of calculated values. A significant discrepancy between theoretical and experimental enthalpy of formation values for L-alanine provides a reason to reconsider the experimental data previously used to derive the standard molar enthalpy of formation Of L-alanine in the gaseous phase at 298.15 K. To obtain a more reliable value of enthalpy of sublimation at 298.15 K, the heat capacity values of gaseous L-alanine were calculated by standard statistical thermodynamics formulae using molecular parameters determined from B3LYP/cc-pVTZ calculations. With the obtained value of C degrees(p.m) (L-alanine, g, 298.15 K) = 112.6 +/- 4.0 J.K-1.mol(-1) the original published experimental values of enthalpy of sublimation of L-alanine were readjusted to the reference temperature: Delta H-g(cr)m (L-alanine, 298.15 K) = 135.2 +/- 2.0 kJ.mol(-1). This value, together with the experimental enthalpy of formation of solid L-alanine, Delta degrees H-r(m) (L-alanine, cr, 298.15 K) = -560.0 +/- 1.0 kJ.mol(-1) [S.N. Ngauv, R. Sabbah, M. Laffitte, Thermochim. Acta 20 (1977) 371-380; I. Contineanu, D.I. Marchidan, Rev. Roum. Chim. 29 (1984) 43-48], gives a new value for the enthalpy of formation Of L-alanine in the gaseous phase, Delta degrees H-f(m) (L-alanine, g, 298.15 K) = -424.8 +/- 2.0 kJ.mol(-1) which is in good agreement with our theoretical G3X result, -427.6 +/- 4.0 kJ.mol(-1). The same procedure for glycine allowed us to improve the literature value of the enthalpy of formation for this compound, Delta degrees H-f(m) (glycine, g, 298.15 K) = -393.7 +/- 1.5 kJ.mol(-1). As a result a set of self-consistent thermochemical data for glycine and L-alanine is proposed. (C) 2008 Elsevier Ltd. All rights reserved.