The solid-solid phase transitions in the perovskite type layer materials(n-C_(12)H_(25)NH_3)_2ZnCl_4 (C_(12)Zn) and (n-C_(18)H_(37)NH_3)_2ZnCl_4 (C_(18)Zn) that are onekind of potential thermal storage material, were synthesized and, at the same time, a series oftheir mixtures C_(12)Zn/C_(18)Zn were prepared. The experimental binary phase diagram ofC_(12)Zn/C_(18)Zn was established by means of differential scanning calorimetry (DSC) and X-raydiffraction. In the phase diagram a stable solid compound(n-C_(12)H_(25)NH_3)(n-C_(18)H_(37)NH_3)ZnCl_4(C_(12)C_(18)Zn) and two eutectoid invariants wereobserved. It is noticeable that the phase diagram contains solid solution ranges.dodecylammonium