The vapor-liquid equilibria and solid phase of the mixed aqueous solutions of sodium and potassium fluorides were studied at different temperatures from 298.15 to 353.15 K. The water activities of {yKF + (1 - y)NaF}(aq) at various ionic-strength fractions y with y = 0.20, 0.50, and 0.80 of KF were measured from dilute to saturation of the solution using the hygrometric method. From these measurements, the related thermodynamic properties were determined using the developed ion interaction model. The mixing ternary parameters theta(NaK) and psi(NaKF) of the ternary system were evaluated and used with the ionic parameters of the pure electrolytes to predict the activity and osmotic coefficients at different temperatures. From these results, the behaviors of the mixed KF-NaF in aqueous solutions were described. Indeed, the activity coefficient values could indicate relatively the presence of the ionic association phenomenon manifested in the aqueous solutions. In fact, as the solution becomes more concentrated, the ions are associated to form an ion pairing. The solubility measurements were also performed at various ionic-strength fractions y. The mineral phases formed were detected by X-ray diffraction and show only a single crystalline form of NaF(s).