The present study includes measurements of the phase composition, density, and refractive index of aqueous two-phase systems of poly(ethylene oxide) 1500 (PEO1500) + sodium thiosulfate + H2O at (283.15, 298.15, and 313.15) K. With the experimental results obtained, phase diagrams were established, and the influence of temperature on the densities, binodal position, and refractive index values was analyzed and discussed. The experimental tie-lines were described using the Othmer-Tobias correlation. The experimental aqueous two-phase system data were correlated with the nonrandom two-liquid (NRTL) thermodynamic model for the activity coefficients; the results were considered excellent according to a global root-mean-square deviation of 0.97 %.