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Heat capacity for 1-hexyl-3-methyl imidazolium perrhenate ionic liquid [C6MIM][ReO4] in the temperature range from 79 to 396 K has been measured by a fully automated adiabatic calorimeter. For [C6MIM][ReO4], glass transition temperature, the melting temperature, standard molar heat capacity, enthalpy and entropy of solid–liquid phase transition were determined to be (202.164 ± 0.405) K, (226.198 ± 0.265) K, (480.702 ± 0.013) J K−1 mol−1, (15.665 ± 0.195) kJ mol−1 and (69.250 ± 0.780) J K−1 mol−1, respectively. In addition, the thermodynamic characteristics and solid–liquid phase change behavior of [C6MIM][ReO4] were compared with the ones of [C7MIM][ReO4] reported in the literature. The thermodynamic functions (HT− H298.15), (ST− S298.15) and (GT− G298.15), for the compound in the experimental temperature range were calculated.

The ionic liquid 1-(2-methoxyethyl)-3-ethyl imidazolium perrhenate [C22O1IM][ReO4] was prepared in the laboratory firstly, and its structure was confirmed by 1H NMR and 13C NMR. The heat capacities were precisely measured in the temperature range from 78 to 392 K by means of a fully automated adiabatic calorimeter. For [C22O1IM][ReO4], the melting temperature, standard molar heat capacity, molar enthalpy, and molar entropy of solid–liquid phase transition were determined to be (211.826 ± 0.130) K, (142.31 ± 0.34) J K−1 mol−1, (14.382 ± 0.046) kJ mol−1, and (67.93 ± 0.22) J K−1 mol−1, respectively. And the experimental values of molar heat capacities were fitted to a polynomial equation using least square method in the appropriate temperature ranges. The thermodynamic functions (HT − H298.15) and (ST − S298.15) were also obtained from the heat capacity data in the experimental temperature range with an interval of 5 K.