The molalities b of (b(1)NH(4)Cl.MgCl2.6H(2)O + b(2)NH(4)Br.MgBr2.6H(2)O)(aq) have been investigated at the temperature T = 348.15 K and formation of a discontinuous series of mixed crystals has been established. The results obtained on the co-crystallization of isostructural chlorocarnallites and bromocarnallites are summarized and the effect of the halide anion (X(-)) and the metal cation (M(+)) on the isomorphism of M(+)X(-).MgX(2).6H(2)O carnallite-type double salts has been evaluated.