This paper reports an experimental and theoretical study of the gas phase standard (p(o) = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, of alpha-alanine (DL) and beta-alanine. The standard (p(o) = 0.1 MPa) molar enthalpies of formation of crystalline alpha-alanine (DL) and beta-alanine were calculated from the standard molar energies of combustion, in oxygen, to yield CO2(g), N-2(g), and H2O(l), measured by static-bomb combustion calorimetry at T = 298.15 K. The vapor pressures of both amino acids were measured as function of temperature by the Knudsen effusion mass-loss technique. The standard molar enthalpies of sublimation at T = 298.15 K was derived from the Clausius-Clapeyron equation. The experimental values were used to calculate the standard (p(o) = 0.1 MPa) enthalpy of formation of alpha-alanine (DL) and beta-alanine in the gaseous phase, Delta H-f(m)o(g), as -426.3 +/- 2.9 and -421.2 +/- 1.9 kJ.mol(-1), respectively. Standard ab initio molecular orbital calculations at the G3 level were performed. Enthalpies of formation, using atomization reactions, were calculated and compared with experimental data. Detailed inspections of the molecular and electronic structures of the compounds studied were carried out.