The solubility of K2SO4 in the mixed solvent, (water (1) + ethanol (2)) with mass fraction w(1) = 0.9, was determined by using the evaporating method in the presence of various concentrations, (0, 0.100, 0.150, 0.200, 0.300, 0.400, 0.500, 0.600, 0.700, and 0.800) mol.L-1, of NaNO3 at 25 degrees C. The values of solubility and solubility product constant were evaluated at zero ionic strength for ions in the saturated solution (at a molarity of NaNO3 = 0 mol.L-1) by using the Van Rysselberahe and Eisenberg model, and the ion-pairing extension was calculated in the considered solution by an extrapolating method that uses the experimental results and estimates the mean activity coefficient at 25 degrees C.