A solid-liquid equilibrium (SLE) study was conducted in the ternary system KNO3 + NH4NO3 + H2O using the isothermal solution saturation method at temperatures of 293.15, 303.15, 313.15, and 323.15 K. The experimental data obtained were used to plot a section of the solubility polytherm of the investigated system and show the dependence of the densities of equilibrium solutions on their chemical compositions. It has been found that a complex salt of the formula K-0.25(NH4)(0.75)NO3 is formed in the system at a temperature below 303.15 K.