The phase diagrams, densities, and refractive indices of a ternary system comprising polyethylene glycol (PEG) 1000, potassium chloride, and water at 288.2, 298.2, and 308.2 K were determined. Only one liquid phase was obtained for the KCl-PEG1000 (PEG of molecular weight 1000)-H2O ternary system over the entire PEG1000 composition range studied. The results revealed that the solubility of KCl increased with an increase in temperature at constant PEG concentration. Conversely, at constant temperature, the solubility of KCl decreased with an increase in PEG1000 in the solution. Moreover, the density of the solution decreased with the addition of PEG1000 in the solution, while the refractive index increased with the increase in PEG1000. The equilibrium thermodynamics of the KCl-PEG1000-H2O system at 288.2, 298.2, and 308.2 K were calculated from the modified Pitzer equation. Finally, a comparison of the experimental and calculation diagrams revealed that the predictive solubilities agree well with the experimental values.