In this work, the molar enthalpies of formation of the ionic liquids [C2MIM][NO3] and [C4MIM][NO3] were measured by means of combustion calorimetry. The molar enthalpy of fusion of [C2MIM] [NO3] was measured using differential scanning calorimetry. Ab initio calculations of the enthalpy of formation in-the gaseous phase have been performed for the ionic species using the G3MP2 theory. We have used a combination of traditional combustion calorimertry with modern high-level ab initio calculations in order to obtain the molar enthalpies of vaporization of a series of the ionic liquids under study.