The binary phase diagram of KNO3-KClO3 is studied by means of differential scanning calorimetry (DSC) and high-temperature X-ray diffraction. The limited solid solutions, K(NO3)(1-x)(ClO3)(x) (0 < x < 0.20) and K(NO3)(1-x)(ClO3)(x) (0.90 < x < 1.0), were formed in the KNO3-based solid solutions and KClO3-based solid solutions phase, respectively. For KNO3-based solid solutions, KNO3 ferroelectric phase can be stable from 423 to 223 K as a result of substituting of NO3 by ClO3-radicals. The temperatures for solidus and liquidus have been determined based on limited solid solutions. Two models, Henrian solution and regular solution theory for KNO3-based (alpha) phase and KClO3-based (beta) 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 alpha and beta 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. (C) 2003 Elsevier B.V. All rights reserved.