Abstract: The solid-solid phase transitions in the perovskite type layer materials (n-C₁₂H₂₅NH₃)₂ZnCl₄ (C₁₂Zn) and (n-C₁₈H₃₇NH₃)₂ZnCl₄ (C₁₈Zn) that are one kind of potential thermal storage material, were synthesized and, at the same time, a series of their mixtures C₁₂Zn/C₁₈Zn were prepared. The experimental binary phase diagram of C₁₂Zn/C₁₈Zn was established by means of differential scanning calorimetry (DSC) and X-ray diffraction. In the phase diagram a stable solid compound (n-C₁₂H₂₅NH₃)(n-C₁₈H₃₇NH₃)ZnCl₄ (C₁₂C₁₈Zn) and two eutectoid invariants were observed. It is noticeable that the phase diagram contains solid solution ranges.