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 tetraalkyl ammonium ions Delta H-f degrees(m) ([R4N](+)((aq))) derived in this work are useful for prediction thermochemical properties of the ammonium based ionic liquids. (C) 2012 Elsevier Ltd. All rights reserved.