The temperature dependences of the permittivity, the amplitude of the third harmonic and the differential thermal analysis signal for the (KNO3)(1–x)/(RbNO3)x composite were studied. To obtain (KNO3)(1–x)/(RbNO3)x samples, KNO3 and RbNO3 powders with particle size of 5–10 µm were used. The powders were mixed in appropriate proportions, after that the tablets were made at a pressure of 8·103 kg/cm2. The samples with x from 0.05 to 0.5 were examined (x is the volume fraction of RbNO3). Before measurements, the obtained samples were heated to 593 K, which is higher than the melting point of RbNO3, but lower than that of KNO3. It has been found that an increase in the RbNO3 content leads to the appearance of additional phase transitions and the expansion of the temperature region of the ferroelectric phase. The transition temperatures for potassium nitrate do not shift along the temperature axis. For rubidium nitrate, the phase transition at 437 K decreases by 4–5 K, and the temperature hysteresis of this transition increases. The expansion of the ferroelectric phase upon cooling is explained by the superposition of the ferroelectric phases of potassium nitrate and the RbxK(1–x) NO3 solid solution.