The simple equations developed for prediction of physical properties of multicomponent aqueous solutions of traditional salts were investigated to see if they still work when ionic liquids were used (as solutes) in place of traditional salts. The densities, conductivities, and viscosities of the ternary system [P-4,P-4,P-4,P-4]Br (tetrabutylphosphonium bromide) + [P-2,P-4,P-4,P-4]Br (ethyltributylphosphonium bromide) + H2O and its binary subsystems [P-4,P-4,P-4,P-4]Br + H2O and [P-2,P-4,P-4,P-4]Br + H2O were measured at 298.15 K, 303.15 K, and 308.15 K, respectively. The measured values were compared with the values predicted by the above-mentioned equations. The comparison results indicate that with the use of these equations the densities, conductivities, and viscosities of [P-4,P-4,P-4,P-4]Br + [P-2,P-4,P-4,P-4] Br + H2O can be predicted from the corresponding data of [P-4,P-4,P-4,P-4]Br + H2O and [P-2,P-4,P-4,P-4]Br + H2O under the condition of identical ionic strength.