The isopiestic method has been used to determine the osmotic coefficients of the binary solutions Rb2SeO4(aq) at the temperature T = 298.15 K from (1.072 to 5.200) mol . kg(-1). The molalities in of (m(1)Rb(2)SeO(4) + m(2)CoSeO(4)) (aq) have been investigated by the physicochemical analysis method. The crystallization of a new double salt Rb2SeO4 . CoSeO4 . 6H(2)O has been established. This double salt crystallizes in a monoclinic crystal system, space group P2(1)/a, a = 0.9281(2) nm, b = 1.2422(2) mn, c = 0.6276(9) nm, beta = 105.70 (2)degrees, and V = 0.6966(2) nm(3). The double salt is isostructural with Rb2SeO4 . ZnSeO4 . 6H(2)O. T.g. and d.t.a measurements indicate that the double salt loses the crystallization water in two steps in the temperature interval from 393.15 K to 523.15 K. The Pitzer ion-interaction model has been used in the thermodynamic analysis of the experimental osmotic and solubility data obtained. The thermodynamic parameters 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)(0) of the double salt Rb2SeO4 . CoSeO4 . 6H(2)O from the corresponding simple salts Rb2SeO4 and CoSeO4.6H(2)O, as well as the standard molar Gibbs energy of formation Delta(f)G(m)(0) have been determined. (C) 2003 Elsevier Science Ltd. All rights reserved.