In the present study, to investigate the soluting effect in ternary aqueous solutions of ionic liquids (ILs), experimental studies have been carried out on ionic liquid 1 + ionic liquid 2 + water systems that are in isopiestic equilibrium. The imidazolium-based ionic liquids 1-ethyI-3-methyl-imidazofium bromide ([C2MIM]Br), 1-butyl-3-methylimidazolium bromide ( [C4MIM]Br), and 1-hexy1-3-methylimidazolium bromide ([C6MIM]Br) were selected, and the vapor liquid equilibria (VLE) of {[C2MIM]Br + [C6MIM]Br + water}, {[C2MIM]Br + [C4MIM]Br + water}, and {[C4MIM]Br + [C6MIM]Br + water} were investigated by the isopiestic method. The equations derived from the Zdanovskii-Stokes-Robinson rule were applied to predict the thermodynamic properties (density, speed of sound, and surface tension) of ternary solutions from the corresponding data of binary subsystems with the same water activity data, and the calculated thermodynamic properties were compared to the experimental data.