Poly(ethylene glycol-ran-propylene glycol) lies within a family of thermo-separating polymers (commercialized as “UCON”), thus they can be easily recovered from aqueous solution just heating above its lower critical solution temperature. Phase diagrams for UCON – sulfate salt aqueous two-phase systems (ATPS) have been obtained experimentally in the temperature range 273.15–308.15K by analysis of the equilibrium phases. The binodal curves and tie-lines have been correlated using suitable methods from the literature. The effect of temperature and the cation used in the sulfate salt (sodium, potassium, ammonium) are discussed. Temperature increases the heterogeneous region size while increases the polymer concentration in the polymer-rich phase and reduces the salt concentration in the salt-rich phase. This effect produces the phase inversion at high temperatures. The cation ability for phase splitting in these ATPS increases with the (more negative) Gibbs energy of hydration of the cation: Na+>K+>NH4+.