This paper reports an experimental and computational thermochemical study on three barbituric acid derivatives: 1,3,5-trimethylbarbituric acid or (1,3,5-trimethyl-2,4,6(1H, 3H, 5H) pyrimidinetrione, CAS 7358-61-4), 1,5,5-trimethylbarbituric acid or (1,5,5-trimethyl-2,4,6(1H, 3H, 5H) pyrimidinetrione, CAS 702-47-6), and 1,3,5,5-tetramethylbarbituric acid or (1,3,5,5-tetramethyl-2,4,6(1H, 3H, 5H) pyrimidinetrione, CAS 13566-66-0). Values of standard (p(0) = 0.1 MPa) molar enthalpies of formation in the gas phase at T = 298.15 K have been derived from experiment. Energies of combustion were measured by static bomb combustion calorimetry and standard molar enthalpies of formation in the crystalline state at T = 298.15 K were calculated. Enthalpies of sublimation were derived from temperature dependence of vapour pressures measured by transpiration method. From these results, values of -(570.6 +/- 2.3), -(599.4 +/- 1.4), and -(605.1 +/- 2.9) kJ . mol(1) for the gas-phase enthalpies of formation at T = 298.15 K of 1,3,5-trimethylbarbituric acid, 1,5,5-trimethylbarbituric acid, and 1,3,5,5-tetramethylbarbituric acid, respectively, were determined. Theoretical calculations at the G3 and G4 levels were performed, and a study of the molecular and electronic structure of the compounds has been carried out. Calculated enthalpies of formation were in good agreement with the experimental values. (C) 2014 Elsevier Ltd. All rights reserved.