The osmotic coefficients of the mixed electrolyte solution {yNaH(2)PO(4) + (1 - y)Na2SO4}(aq) have been measured by the isopiestic method, at the temperature T = 298.15 K. The activity coefficients of NaH2PO4 and Na2SO4 were calculated by Scatchard's neutral-electrolyte method and by Pitzer and Kim's treatment for mixed-electrolyte solutions. The Scatchard interaction parameters are used for calculation of the excess Gibbs energy as a function of ionic strength and ionic-strength fraction of NaH2PO4. Also, the Zdanovskii's rule of linearity is tested.