The solutions (m(1)MgSeO(4) + m(2)NiSeO(4))(aq), (m(1)MgSeO(4) + m(2)CoSe(4))(aq), (m(1)ZnSeO(4) + m(2)NiSeO(4))(aq) and (m(1)ZnSeO(4) + m(2)CoSeO(4))(aq), where m denotes molality, have been simulated thermodynamically on the basis of the Fitter model at the temperature T = 298.15 K. The thermodynamic functions needed (binary and ternary ion-interaction parameters and thermodynamic solubility products) have been determined and the standard molar Gibbs free energy of mixing Delta(mix)G(m) degrees(s) Of the (Me,Me’)SeO4 . 6H(2)O crystals has been calculated by two different methods. The results obtained are compared with the experimental data available in the literature. The reasons for the observed differences are given.