This paper reports an experimental and computational thermochemical study on 1,3-dimethylbarbituric acid. The value of the standard (p(o) = 0.1 MPa) molar enthalpy of formation in the gas phase at T = 298.15 K has been determined. The energy of combustion was measured by static bomb combustion calorimetry, and from the result obtained, the standard molar enthalpy of formation in the crystalline state at T = 298.15 K was calculated as -639.6 +/- 1.9 kJ.mol(-1). The enthalpy of sublimation was determined using a transference (transpiration) method in a saturated N-2 stream and a value of the enthalpy of sublimation at T = 298.15 K was derived as 92.3 +/- 0.6 kJ.mol(-1). From these results a value of -547.3 +/- 2.0 kJ.mol(-1) for the gas-phase enthalpy of formation at T = 298.15 K was determined. Theoretical calculations at the G3 and G4 levels were performed, and a study on molecular and electronic structure of the compound has been carried out. Calculated enthalpies of formation are in very good agreement with the experimental value.