Activity coefficients for potassium chloride in the KCl + K2SO4 + H2O ternary system were determined from electromotive force (emf) measurements of the cell: K ion selective electrode (ISE) \KCl (M-A), K2SO4 (m(B)), H2O \Ag\AgCl at 298.15 K and over total ionic strengths from 0.01 mol(.)kg(-1) up to the saturated concentration 2.3 mol(.)kg(-1). The K+ ion selective electrode (K-ISE) and Ag\AgCl electrode used in this work were prepared in our laboratory and had reasonably good Nernst responses, which demonstrate that the emf method can be applied to measure the above system with high precision in comparison with other methods. The experimental data were analyzed using the Harned rule and the Pitzer model. The Harned coefficients and the Pitzer binary and ternary ionic interaction parameters for the system have been evaluated. The experimental results obey the Harned rule, and the Pitzer model can be used to describe this aqueous system satisfactorily. The activity coefficients of K2SO4, the osmotic coefficients of the mixtures, and the excess free energy of mixing were also calculated. Finally, the mutual solubility of the mixed salts was predicted by way of the obtained Pitzer parameters.