In this paper we report the standard (p degrees = 0.1 MPa) molar enthalpy of formation of quinazoline-2,4(1H,3H)-dione, in the gaseous phase, at T = 298.15 K, which was derived from a combined experimental and computational thermochemical work. The static bomb combustion calorimetry technique was used to determine the standard massic energy of combustion and consequently the standard molar enthalpy of formation, in the crystalline phase, at T = 298.15 K, and the Calvet microcalorimetry technique was employed to determine the standard molar enthalpy of sublimation, at T = 298.15 K. The obtained standard molar enthalpy of formation, in the gaseous phase, at T = 298.15 K, was: Delta(f)H(m)degrees(g) = (271.1 +/- 3.3) kJ . mol(-1). In addition, quantum chemical calculations were performed using the G3(MP2)//B3LYP composite method to assess the relative energetic stabilities of quinazoline-2,4(1H,3H)-dione and its tautomers (4-hydroxyquinazolin-2(1H)-one, 2-hydroxyquinazolin-4(1H)-one, 2-hydroxyquinazolin-4(3H)-one, and quinazolin-2,4-diol). Estimates of the standard molar enthalpy of formation, in the gaseous phase, of the most stable tautomer, quinazoline-2,4(1H,3H)dione, at T = 298.15 K, were obtained from the G3(MP2)//B3LYP calculations using appropriate isodesmic reactions and were further compared with the obtained experimental value. The aromaticity of quinazoline-2,4(1H,3H)-dione has also been assessed through the evaluation and analysis of the Nucleus Independent Chemical Shifts (NICS) and their most significant components for the benzene and pyrimidine rings.