Heat capacity for 1-butyl-3-methylimidazolium trifluoromethanesulfonate [C4mim]OTf in the temperature range of (5-370)K has been measured in an adiabatic calorimeter The temperature T-fus = (290 98 +/- 0 01)K and the enthalpy of fusion Delta H-fus(m)o = (19 43 +/- 0 02) kJ mol(-1) were determined A solid-phase transition was observed near T=120 K Thermodynamic functions for the compound in the condensed state were calculated Trends in the solid-liquid phase change entropies Sigma Delta S-tr(m) for 1-alkyl-3-methylimidazolium salts [C(n)mim]An were analyzed The Increment to Sigma Delta S-tr was shown to increase with n It was found that Sigma Delta Sm-tr =66 4 +/- 1 3 J K-1 mol(-1) for all the studied [C(4)mim]An ionic liquids except [C(4)mim]NTf2 This difference can be partly explained by formation of two distinguishable conformers of the NTf2 anion in the liquid state (C) 2010 Elsevier B V All rights reserved