Osmotic coefficient and density measurements are reported for the aqueous solutions of three hydrochloride salts of local anesthetical drug compounds, procaine (PC center dot HCl), lidocaine (LC center dot HCl), and tetracaine (TC center dot HCl) at 298.15 K and at ambient pressure. The experimental osmotic coefficient data are used to determine the activity and mean ionic activity coefficients of solute and solvent, respectively. The activity data have been processed to obtain the mixing and excess thermodynamic properties, such as Gibbs free energy (which has been studied as a function of drug concentration), as well as to obtain the osmotic pressure and osmotic virial coefficients of the drug compounds. The mean ionic activity coefficients of the ions decrease with the increase in drug concentration. The results of mixing and excess free energy changes do not show abrupt changes. These results are examined from the point of view of premiceller (associative) equilibra and the occurrence of critical micelle concentration (cmc). A discussion is presented on the basis of aggregation of cations, and the aggregation numbers of 2, 1.56, and 6 are obtained for PC center dot HCl, LC center dot HCl, and TC center dot HCl, respectively, in the solution phase, applying the pseudophase separation model. An application of the McMillan-Mayer theory of solutions to the data is made. It is noted that overall second virial coefficients are small and negative for the drug molecular salt, whereas it is positive for nonelectrolyte contribution. All of these are examined on the basis of structural characteristic of molecules and electrostatic and hydrophobic interactions.