The effect of temperature on the liquid-liquid equilibrium for the aqueous two-phase system water + poly(ethylene glycol) 4000 + potassium phosphate was studied by determining 29 tie lines at 10 °C, 14 tie lines at 15 °C, 23 tie lines at 20 °C, and 15 tie lines at 30 °C, to give a total of 81 tie lines at four different temperatures. The results show a transition of miscibility: at 10 °C, the pair water + salt is completely miscible and the pair PEG + water is partially miscible, while, at higher temperatures, the pair water + salt becomes partially miscible and the pair PEG + water becomes completely miscible. Experimental results were correlated with a mass fraction-based NRTL activity coefficient model. New interaction parameters were estimated with the Simplex method and the maximum likelihood principle. The mean deviations between the experimental and calculated compositions in both equilibrium phases are below 1.7%.