The osmotic coefficients of the mixed electrolyte solution {y Na2SO4 + (1 - y)ZnSO4} (aq) have been measured by the isopiestic method, at T = 298.5 K. The experimental results were treated by Scatchard's, Pitzer-Kim's and Clegg-Pitzer-Brimblecombe's methods for mixed-electrolyte solutions. By these methods, the activity coefficients for Na2SO4 and ZnSO4 were calculated and compared. The Scatchard interaction parameters are used for calculation of the excess Gibbs free energy as a function of ionic strength and ionic-strength fraction of Na2SO4. Also, the Zdanovskii's rule of linearity is tested. (C) 2004 Elsevier Ltd. All rights reserved.