The osmotic coefficient measurements have been carried out for ternary aqueous solutions containing a fixed concentration of alpha-cyclodextrin (a-CD) of similar to 0.1 mol . kg(-1) and varying the concentrations (similar to 0.012 to similar to 0.21 mol . kg(-1)) of drugs Procaine hydrochloride (PC .HCl), Lidocaine hydrochloride (LC . HCl), Tetracaine hydrochloride (TC . HCl) and Ranitidine hydrochloride (RT . HCl) at 298.15 K using vapour pressure osmometry. The water activities for each ternary system were measured and used to obtain the activity coefficients of a-cyclodextrin (alpha-CD) and drugs following the methodology developed by Robinson and Stokes for isopiestic measurements. The transfer Gibbs free energies of electrolyte (or drug) from water to an aqueous nonelectrolyte (alpha-CD) solutions (DGE tr) and that of nonelectrolyte (alpha-CD) from water to an aqueous electrolyte (or drug) solutions (DGN tr) have been calculated using the activity data. These were further used for the estimation of pair and triplet interaction parameters. By applying the method based on the application of the McMillan-Mayer theory of virial coefficients to transfer free energy data, the salting constant (ks) values have been estimated at 298.15 K. The equilibrium constant (logK) values for the complexation due to host-guest interaction are found high meaning encapsulation of drug molecules in a-CD cavities accompanied by pair-wise and triplet interaction due to hydrophobic hydration and interaction. The logK values are in the order of TC.HCl > RT . HCl > PC . HCl > LC . HCl. All these results are presented, compared and explained in light of host-guest interaction concepts. (C) 2013 Elsevier Ltd. All rights reserved.