In this work, the thermodynamic properties of the mixed electrolyte system (KCl + KNO3 + water + proline), using the potentiometric method, are reported. The potentiometric measurements were performed on the galvanic cells without liquid junction of the type: Ag-AgCl vertical bar KCl (m(1)), KNO3 (m(2)), (100-wt%) H2O + wt% proline vertical bar K-ISE, NO3-ISE vertical bar KNO3 (m(1)), (100-wt%) H2O + wt% proline 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(KCl)/m(KNO3) = 0.1, 1, 2.5, 5.0, 10.0) and pure KCl in mixed solvent system containing 5% mass fraction of proline at T = 298.2 and 303.2 K. The PVC based potassium and nitrate ion selective electrodes (K, NO3-ISE) and Ag-AgCl electrode used in this work were prepared in our laboratory and had a reasonably Nernst response. The activity coefficients data were interpreted based on the Harned rule and Pitzer model. The unknown Pitzer mixing interaction parameters (theta(ClNO3), psi(KClNO3)) were evaluated for the studied mixed electrolyte system according to Pitzer graphical method. Then, the parameters obtained with the Pitzer model were used to calculate the values of the mean activity coefficients of KNO3, the osmotic coefficients, and the excess Gibbs free energies for the whole series of the studied mixed electrolyte system. (c) 2012 Elsevier B.V. All rights reserved.