The effect of poly(ethylene glycol) (PEG) molar mass, pH, and temperature on interfacial tension was investigated for aqueous two-phase systems composed of PEG with molar masses of (1500, 4000, 6000, and 8000) g.mol(-1) and potassium phosphate. The temperatures tested were (293 and 313) K at the pH values of 6, 7, and 9. Interfacial tension was determined by using a spinning drop tensiometer and the Vonnegut equation. An increase in both the PEG molar mass and the temperature resulted in an increase of interfacial tension values. The interfacial tension varied from (0.05 to 3.11) mN.m(-1) for systems composed by PEG 4000 (mass fraction; w = 0.12) + potassium phosphate (w = 0.11) + water (w = 0.77) at pH 6.0 and 293 K and PEG 4000 (w = 0.19) + potassium phosphate (w = 0.16) + water (w = 0.65) at pH 7.0 and 313 K, respectively.