The goals of this work are (i) to analyze the influence of various factors on the calculated values of the mean-ion activity coefficient and integral properties of ternary solutions and (ii) to determine the main sources of errors in calculated values. The water–ethanol–sodium chloride system was chosen as a test system because data for the ternary system [1], [2], [3], [4] and [5] and the binary subsystem [6] were reported by various authors. The properties of the water–alcohol system need to be considered when describing the integral properties of salt solutions in a mixed water–alcohol solvent. In general, many papers present the results only of experimental data approximation without primary data which are required for the tasks formulated above. As a result additional measurements of EMF in cells with ISE were carried out. The conditions for potentiometric measurements were chosen so as to test the reproducibility of the literature data and to widen the temperature and concentration ranges.