Abstract Three ionic liquids (ILs): 1-butyl-3-methyl-imidazolium hydrogensulfate C4C1ImHSO4, 1-butyl-imidazolium hydrogensulfate C4C0ImHSO4, and 1-methyl-imidazolium hydrogensulfate C1C0ImHSO4, were synthesized and characterized. The effect of alky group incorporation into the imidazolium cation was evaluated in terms of changes in density, surface tension, viscosity, and thermal degradation of the ILs. The viscosity and density were measured within the temperature range of 293.15-373.15 K, and experimental values of density were used to calculate molar volume, standard molar entropy, lattice energy, and thermal expansion coefficients. Glasser's approach was used to calculate the lattice energies of the ILs. Surface tensions in the temperature range of 293.15-353.15 K were also measured and surface tension values were used to estimate the surface entropy and enthalpy of the ILs. Refractive indices were measured with an ATAGO refractometer (RX-5000α) within the temperature range of 293.15-323.15 K. Thermal gravimetric analysis was performed in the temperature range of 323.15-773.15 K. © Springer Science+Business Media New York 2015.