The isopiestic method has been used to determine the osmotic coefficients of the binary solutions Na2SeO4(aq) at the temperature 298.15K from (0.636 to 2.811)mol.kg(-1). The solubilities of (m(1)Na(2)SeO(4) + m(2)NiSeO(4))(aq) where m denotes molality have been investigated by the physico-chemical analysis method. It has been established that the system is of a simple eutonic type. The Fitter ion-interaction model has been used for the thermodynamic analysis of the results obtained. The thermodynamic functions needed (binary and ternary parameters of the ionic interaction and the thermodynamic solubility products) have been calculated and the theoretical solubility isotherm has been plotted. The experimentally obtained and the calculated solubilities are in very good agreement.