The molalities b of (b(1)RbCl . MgCl2 . 6H(2)O + b(2)RbBr . MgBr2 . 6H(2)O)(aq) have been investigated at the temperature T = 323.15 K and formation of a discontinuous series of mixed crystals has been established. On the basis of the present and earlier experimental results, it has been established that the formation of a continuous mixed-crystal series of isostructural carnallite-type double salts depends on close values of the unit-cell parameters. The parameter Delta delta, accounting for the differences in the unit-cell volumes of the participating double salts and their mole fractions in the mixed crystals, is proposed as a quantitative criterion. It has been experimentally proved that the continuous mixed-crystal series is interrupted when Delta delta greater than or equal to 2.5 . 10(-2) In systems where continuous mixed-crystal series are formed, the distribution coefficients correspond to the theoretical values calculated according to the equation of isomorphic co-crystallization. On the basis of this condition, the solubilities in (b(1)RbCl . MgCl2 . 6H(2)O + b(2)CsCl . MgCl2 . 6H(2)O)(aq) at T = 323.15 K are determined theoretically.