The isopiestic method has been used to determine the osmotic coefficients of the binary solutions Cs2SeO4(aq) at T = 298.15 K from (1.090 to 4.591) mol (.) kg(-1). The molalities m of (m(1)Cs(2)SeO(4) + m(2)NiSeO(4)) (aq) have been investigated by physicochemical analysis. The crystallization of a new double salt Cs2SeO4 (.) NiSeO4 (.) 6H(2)O has been established. The Pitzer ion-interaction model has been used for thermodynamic analysis of the results obtained. The thermodynamic data needed (binary and ternary parameters of ionic interaction, 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. The standard molar Gibbs energy of the synthesis reaction Delta(r)G(m)(o) of the double salt Cs2SeO4 (.) NiSeO4 (.) 6H(2)O from the corresponding simple salts Cs2SeO4 and NiSeO4 (.) 6H(2)O, as well as the standard molar Gibbs energy of formation Delta(r)G(m)(o) have been determined.