The activities of chlorpromazine ion (a_{CPZ. H⁺}) and chloride ion (a_Cl^-) were directly obtained by the EMF measurement of a concentration cell separated with cation-exchange membrane and of another concentration cell, which uses Ag-AgCl electrode reversible to Cl^- ion, respectively, for aqueous solutions of chlorpromazine hydrochloride (CPZ·HCl). Transport number of CPZ·H⁺ ion in cation-exchange membrane was not assumed. to be a unity, but found to be t₊=0.82, which was estimated by concentration dependence of EMF of the concentration cell. Ionic activity, activity coefficient, and also their mean ionic quantities were calculated. Below the critical micelle concentration (cmc) both the cationic and anionic activities increased with concentration, followed by the theoretical line expected from the Debye-Huckel theory. Above the cmc, the activity a_CPZ.H⁺ gradually decreased, while a_Cl^- showed an abrupt increase just above the cmc, and then, gradually increased with the concentration. Mean activity was not constant, but slightly increased with the concentration above the cmc. The observed decrease in the surface tension of aqueous CPZ·HCl solutions above the cmc was explained by gradual increase in a_±. The mass action law was suggested to be the probable explanation for the aggregation mechanism.