Vapour pressure osmometery (VPO) measurements at T = 308.15 K for {[C6mim][Cl] + water}, {[C6mim][Cl] + (0.005, 0.0155, and 0.0263) mol kg1 PEG2000 + water} and {[C6mim][Cl] + (0.0017, 0.0052, and 0.0088) mol kg1 PEG6000 + water} systems and isopiestic measurements at T = 298.15 K for {[C6mim][Cl] + PEG2000 + water} and {[C6mim][Cl] + PEG6000 + water} systems have been carried out. The VPO measurements were carried out at very low concentrations of PEG and from which the values of the water activities, osmotic coefficients, vapour pressure and activity coefficients were obtained. The data obtained from the VPO method show that over the whole concentration range of the ionic liquid (IL), the activity coefficients of [C6mim][Cl] in the presence of PEG2000 are increased. Although, at high IL concentrations, the values of the activity coefficient of [C6mim][Cl] in the presence of PEG6000 are also increased, however for low concentrations of IL the values of the activity coefficient of [C6mim][Cl] in pure water are larger than those in aqueous PEG6000 solutions. For a known IL concentration, the values of water activity coefficient for the binary {[C6mim][Cl] + water} system are larger than those for the ternary {[C6mim][Cl] + PEG + water} systems and decrease by increasing the concentration of PEG or decreasing the molar mass of PEG. The constant water activity lines of the all ternary systems obtained from the isopiestic method show positive deviation from the linear isopiestic relation (Zdanovskii– Stokes–Robinson rule) derived using the semi-ideal hydration model. The results have been interpreted in terms of the solute–water and solute–solute interactions.
