In this work, the potentiometric measurements were performed for the ternary (KCl + KNO3 + H2O) systems on the galvanic cell of the type: Ag vertical bar AgCl vertical bar KCl (m(1)), KNO3 (m(2)), H2O vertical bar K-ISE over total ionic strengths from (0.001 to 3.000) mol . kg(-1) for different: series of salt molal ratios r (r = m(1)/m(2) = 0.20, 1.0, 5.0, 10.0, and 20.0). The electrodes used in this work were prepared in our laboratory and had a reasonably good Nernst response. The activity coefficients data were interpreted by using Pitzer model. The unknown Pitzer mixing interaction parameters have been evaluated for the system under investigation (theta Cl,NO3 = 0.0186, psi(KClNO3) = -0.0066). Then, the parameters obtained were used to calculate the values of the mean activity coefficients of KNO3, the osmotic coefficients, the solvent activity, the surface tension, and the excess Gibbs free energies for the whole series of the studied mixed electrolyte systems. (C) 2009 Elsevier Ltd. All rights reserved.