In this work, the thermodynamic properties of the quaternary electrolyte (NaCl + NaNO3 + HCONH2 + H2O) system, using the potentiometric method, are reported. The potentiometric measurements were carried out on the galvanic cells without liquid junction of the types: Ag-AgCl vertical bar NaCl (m(A)), (100-w%) H2O + w% HCONH2 vertical bar Na-ISE, Ag-AgCl vertical bar NaCl (m(1)), NaNO3 (m(2)), (100-w%) H2O + w% HCONH2 vertical bar Na-ISE and NO3-ISE vertical bar NaNO3 (m(B)), (100-w%) H2O + w% HCONH2 vertical bar Na-ISE over total ionic strengths from 0.001 to 4.000 mol . kg(-1). Different series of the salt molal ratios r (r = m(NaCl)/m(NaNO3) = 0.1, 1, 2.5, 5.0, 10.0) and pure NaCl in mixed solvent system containing 10% mass fraction of formamide were used for the measurements at T = (298.2 and 303.2) K. The modeling of this quaternary system was made based on the Pitzer ion-interaction model. In according to Pitzer graphical method, the mixed ionic interaction parameters (theta(ClNO3), psi(NaClNO3)) were evaluated for each series of the studied system. Then, the obtained parameters were used to calculate the values of the mean activity coefficients of NaNO3, the osmotic coefficients, solvent activity and the excess Gibbs free energies for the whole series of the studied mixed electrolyte system at T = (298.2 and 303.2) K. (C) 2012 Elsevier Ltd. All rights reserved.