Activity coefficients of the components in aqueous ternary solutions containing a fixed concentration of alpha-cyclodextrin and 18-crown-6 at various concentrations of the third solutes potassium acetate and hydroquinone, respectively (concentration range (0.0 to 0.20) mol . kg(-1)), at 298.15 K were derived from vapor pressure osmometry. Also, data for the osmotic coefficients and activity coefficients for aqueous binary solutions of potassium acetate are obtained and used along with the data for aqueous binary hydroquinone solutions reported earlier to analyze the data of the ternary mixtures. It is observed that there is a lowering of activity coefficients of potassium acetate as well as of hydroquinone in the presence of a-cyclodextrin and 18-crown-6, respectively, which is being attributed to incorporation of the acetate ion and hydroquinone into the (x-cyclodextrin and 18-crown-6 cavities, respectively, to form 1:1 inclusion complexes in the solution phase. The salting-in and thermodynamic equilibrium constant values have been determined using calculated transfer free energies and the method based on application of the McMillan-Mayer virial coefficient theory for solutions. It is noted that the complexation process is assisted by hydrophobic interaction between the complexed species. The apparent molar volume and calculation of virial coefficient for potassium acetate in binary aqueous solutions at 298.15 K are also reported and discussed.