The hygrometric method is used for determination of the relative humidity of aqueous mixed electrolyte systems with magnesium and sodium sulphates. The relative humidities of the system (yMgSO(4) + (1 - y)Na2SO4)(aq) has been measured at total molalities from 0.2 mol kg(-1) to about saturation of one of the solutes for different ionic-strength fractions of MgSO4 with y = 0.2, 0.5 and 0.8 at the temperature 298.15 K. The obtained data allow the deduction of the water activities and osmotic coefficients. The experimental results are compared with the predictions of the extended composed additivity (ECA) law, and the Zdanovskii-Stokes-Robinson (ZSR), Pitzer, and Lietzke and Stoughton (LS II) models. From these measurements, new Pitzer mixed ionic parameters are determined and used to predict the solute activity coefficients in the mixture. The obtained results are used to calculate the excess Gibbs energy at total molalities for ionic-strength fractions y.