The enthalpy of the 1-butyl-3-methylimidazolium iodide synthesis, its heat capacity and parameters of fusion were determined from calorimetric measurements The standard molar enthalpy of formation for this compound in the crystal and liquid states at T = 298 K was determined from these results to be -(138 +/- 6) kJ mol(-1) and -(119 +/- 6) kJ mol(-1), respectively With the use of the literature data and the results obtained in this work, a simple equation for calculation of Delta H-f(m)degrees for 1-butyl-3-methylimidazolium compounds in the crystalline and liquid states is proposed It was demonstrated that the additive scheme by Domalski and Hearing (1993) can be used for calculation of Delta H-f(m)degrees in the homologous series of ionic liquid forming compounds in the liquid state However, for the crystalline state, all the analyzed additive schemes did not provide satisfactory description of the experimental data (C) 2010 Elsevier Ltd All rights reserved