In the mixed electrolyte solution {xK(2)SO(4) + (1-x)MgSO4}(aq), the osmotic coefficients have been measured by the isopiestic method at the temperature 298.15 K. Scatchard’s neutral-electrolyte method and Pitzer-Kim’s treatment were used for the activity coefficients of the K2SO4 and MgSO4 calculation. The results obtained are used for the calculation of the excess molar Gibbs free energy as a function of ionic strength and ionic-strength fraction of K2SO4, and also for the test of Zdanovskii’s rule.