This work confirms that the activities of individual ions measured with ion-selective electrodes are physically meaningful. The individual activities of the Cl-, Na+ and K+ ions in single-electrolyte aqueous solutions of HCl, NaOH and KOH were measured at 298.2 K in the range from 0 to 2 molal. In the pH range from 1 to 13, the pH values calculated from the measured activities of the ions are in good agreement with the values obtained with a pH probe. In agreement with previous results for potassium ions, and opposite to the behavior of other cations in 1:1 electrolyte solutions, the K+ and the H+ ions were found to have smaller activities than their conjugate anions. The experimental activity, coefficients of the ions were con-elated with the Khoshkbarchi-Vera equation and with the New Hydration Theory, and compared with the predictions given by the Pitzer theory.