In this work data of the molar enthalpies of formation of the ionic liquids [R4N][NO3] were measured by means of combustion calorimetry. The thermal behavior of [R4N][NO3] was studied using differential scanning calorimetry. First-principles calculations of the enthalpy of formation in the gaseous phase have been performed for the ionic species using the G3(MP2) theory. Enthalpies of formation of aqueous tetra-alkyl ammonium ions ([R4N]+(aq)) derived in this work are useful for prediction thermochemical properties of the ammonium based ionic liquids.