A thermochemical and thermophysical study has been carried out for crystalline barbital [5,5'-diethyl-barbituric acid]. The thermochemical study was made by static bomb combustion calorimetry, from which the standard (p degrees = 0.1 MPa) molar enthalpy of formation of the crystalline barbital, at T = 298.15 K, was derived as -(753.0 +/- 1.8) kJ . mol(-1). The thermophysical study was made by differential scanning calorimetry over the temperature interval (265 to 470) K. A solid-solid phase transition was found at T = 413.3 K. The vapour pressures of the crystalline barbital were measured at several temperatures between T = (355 and 377) K, by the Knudsen mass-loss effusion technique, from which the standard molar enthalpy of sublimation, at T = 298.15 K was derived as (117.3 +/- 0.6) kJ . mol(-1). The combination of the experimental results yielded the standard molar enthalpy of formation of barbital in the gaseous phase, at T = 298.15 K, as -(635.8 +/- 1.9) kJ . mol(-1). This value is compared and discussed with our theoretical calculations by several methods (Gaussian-n theories G2 and G3, complete basis set CBS-QB3, density functional B3P86 and B3LYP) by means of atomization and isodesmic reaction schemes. (C) 2009 Elsevier Ltd. All rights reserved.