Abstract <p>The phase equilibria and solubility in component mixtures were studied by the visual polythermal method in ten sections of the composition triangle in the temperature range of 10–110°C for the cesium nitrate–water–polyethylene glycol 1500 ternary system. Using the method of the volume ratio of liquid phases, the temperature of formation of the critical node of the monotectic state (78.8°C) and the temperature dependence of the compositions of solutions corresponding to the critical solubility points of the phase separation region were determined. The isothermal phase diagrams for this ternary system were constructed at 10.0, 25.0, 40.0, 50.0, 78.8, 90.0, and 100.0°C, and the solubility of the components was determined. It was found that in the range of 10.0–40.0°C, a triangle for the eutonic state exists in the isothermal diagrams. Above the phase separation onset temperature (78.8°C), a monotectic triangle with adjacent saturated solution and phase separation fields is present in the isotherms. The partition coefficients of polyethylene glycol 1500 between the equilibrium liquid phases of the monotectic state were calculated in the range of 78.8–100.0°C. It was shown that cesium nitrate is effective as a salting-out agent for polyethylene glycol 1500 at temperatures above 90.0°C. At all temperatures of the indicated range, polyethylene glycol 1500 considerably decreases the solubility of cesium nitrate in water.</p>