Abstract The binary phase diagram of KNO 3 –KClO 3 is studied by means of differential scanning calorimetry (DSC) and high-temperature X-ray diffraction. The limited solid solutions, K(NO 3 ) 1− x (ClO 3 ) x (0 x 3 ) 1− x (ClO 3 ) x (0.90 x 3 -based solid solutions and KClO 3 -based solid solutions phase, respectively. For KNO 3 -based solid solutions, KNO 3 ferroelectric phase can be stable from 423 to 223 K as a result of substituting of NO 3 by ClO 3 -radicals. The temperatures for solidus and liquidus have been determined based on limited solid solutions. Two models, Henrian solution and regular solution theory for KNO 3 -based (α) phase and KClO 3 -based (β) phase, respectively, are employed to reproduce solidus and liquidus of the phase diagram. The results are in good agreement with the DSC data. The thermodynamic properties for α and β solid solutions have been derived from an optimization procedure using the experimental data. The calculated phase diagram and optimized thermodynamic parameters are thermodynamically self-consistent.