Thermodynamic parameters at infinite dilution for thirty-one organic solutes in a series of imidazolyl hydrogen sulfate ionic liquids (ILs) were determined by inverse gas chromatography (IGC). The IGC characterization was carried out at temperatures ranging between 303.15 and 343.15 K for determining the Hildebrand solubility parameter (δ 2 ) as well as the Flory−Huggins interaction parameter (χ 12 ∞ ), the activity coefficient at infinite dilution (γ 12 ∞ ), and the thermodynamic functions. In addition, based on the γ 12 ∞ results, the selectivity and capacity were directly calculated for two separation problems (hexane/benzene and hexane/thiophene). Under the different effects of the three ILs, it was found that the values of χ 12 ∞ and γ 12 ∞ were significantly different for the solutes tested, but all the values follow a certain pattern: they increase with temperature or decrease with temperature. As the temperature increased, the solubility parameters of the three ILs showed a linear decrease trend. The solubility parameters of the three ionic liquids at room temperature were obtained by extrapolation as δ 2([BMIM][HSO4]) = 24.15 MPa 1/2 , δ 2([HMIM][HSO4]) = 24.10 MPa 1/2 , and δ 2([OMIM][HSO4]) = 23.66 MPa 1/2 .