The osmotic coefficients of the mixed electrolyte solution {xKNO(3) + (1 - x)MgSO4}(aq) have been measured by the isopiestic method at the temperature 298.15 K. The activity coefficients of KNO3 and MgSO4 were calculated by Scatchard’s neutral-electrolyte method and by Fitter and Kim’s treatment for mixed-electrolyte solutions. The results are used for calculation of the excess Gibbs free energy as a function of ionic strength and ionic-strength fraction of KNO3 and also for a test of Zdanovskii’s rule of linearity.