Phase diagrams of aqueous two-phase systems (ATPS) composed of 400 g.mol(-1) poly(ethylene glycol) (PEG) + sodium sulfate or magnesium sulfate were determined at (298.15, 308.15, and 318.15) K. The temperature effect oil the biphasic area was not relevant, indicating a small enthalpic contribution associated with the phase separation. The ability of the salts in inducing the formation of the ATPS with PEG 400 followed the order sodium sulfate < magnesium sulfate. The liquid-liquid equilibrium experimental data were correlated using the NRTL (nonrandom two-liquid) activity Coefficient Model, and new interaction parameters were estimated. The mean deviations between calculated and experimental compositions were lower than 0.99 %, showing the good descriptive quality and applicability of the NRTL model.