In this work the molar enthalpy of formation of the ionic liquid 1-ethyl-3-methylimidazolium dicyanoamide in the gaseous phase [C₂MIM][N(CN)₂] was measured by means of combustion calorimetry and enthalpy of vaporization using transpiration. Available, but scarce, primary experimental results on enthalpies of formation of imidazolium based ionic liquids with the cation [C_nMIM] (where n = 2 and 4) and anions [N(CN)₂], [NO₃] and [NTf₂] were collected and checked for consistency using a group additivity procedure. First-principles calculations of the enthalpies of formation in the gaseous phase for the ionic liquids with the common cation [C_nMIM] (where n = 2 and 4) and with the anions [N(CN)₂], [NO₃], [NTf₂], [Cl], [BF₄] and [PF₆] have been performed using the G3MP2 theory. It has been established that the gaseous phase enthalpies of formation of these ionic liquids obey the group additivity rules.