The standard (p(0) = 0.1 MPa) molar energies of combustion, Delta U-c(m)0, for the crystalline 5-isopropylbarbituric and 2-thiobarbituric acids were determined, at the temperature of 298.15 K, by static bomb or rotating bomb combustion calorimetry, respectively. For 5-isopropylbarbituric acid, the standard molar enthalpy of sublimation, Delta H-g(cr)m(0), at T=298.15 K, was derived by Calvet microcalorimetry. The standard molar enthalpy of sublimation, at T=298.15 K, was derived by the Clausius-Clapeyron equation, from the temperature-vapor pressure dependence, measured by the Knudsen mass-loss effusion method. These experiments allowed the determination of the standard molar enthalpy of formation, in the gaseous phase, at T=298.15 K for 5-isopropylbarbituric acid. For 2-thiobarbituric acid, the gas-phase enthalpy of formation was calculated combining the result derived for the crystalline phase with literature data for the enthalpy of sublimation, at T=298.15 K. These values were compared with estimates obtained from very accurate computational calculations using the G3 and G4 level theory composite methods. (C) 2015 Elsevier B.V. All rights reserved.