The molalities m of (m(2)2RbCl . NiCl2 . 2H(2)O + m(2)2RbCl . MnCl(2)2H(2)O)(aq) have been investigated at the temperature T = 298.15 K and formation of a continuous series of mixed crystals has been found. On the basis of the present and earlier experimental data it has been established that the formation of a continuous mixed crystal series of isostructural double salts 2MCl . MeCl(2) . 2H(2)O depends on the close values of the unit cell parameters. The basic Fitter ion-interaction model has been used for the first time for determining the thermodynamic characteristics of the formation process of mixed crystals of double salts. The component activities of the saturated ternary solution (m(1)2RbCl . NiCl . 2H(2)O + m(2)2RbCl . MnCl2 . 2H(2)O) (aq) and in the mixed crystalline phase are determined and the change of the molar Gibbs energy of mixing Delta(mix)G(m) of crystals is determined depending on the solid phase composition given in mole fractions. It is established that at T = 298.15 K the mixed 2RbCl .(Ni, Mn)Cl-2 . H2O crystals show positive deviations from the ideal mixed crystals. Using the regular mixing model, the results obtained can be summarized by the following equation : Delta(mix)G(m) = -x(1)x(2) .(3.30 +/- 0.50) kJ . mol(-1).