Surface excess enthalpy, Delta H(sigma), associated with the interfacial formation process was determined at 298.15 K for five different aqueous two-phase systems (ATPS): L35 + Li(2)SO(4) + H(2)O, PEO1500 + Na(2)SO(4) + H(2)O, PEO1500 + Li(2)SO(4) + H(2)O, PEO1500 + citrate + H(2)O, and PEO4000 + Na(2)SO(4) + H(2)O. Where L35 is one triblock copolymer denominated poly(oxide ethylene)-block-poly(oxide propylene)-blockpoly(oxide ethylene) with molar mass 1900 mol.g(-1) and 50 % EO, PEO1500 is poly(oxide ethylene) with molar mass 1500 mol.g(-1) and PEO4000 is poly(oxide ethylene) with molar mass 4000 mol.g(-1). The results show that the interface formation process could be exothermic or endothermic depending on the aqueous two-phase systems and/or tie line length (TLL), and its value is in the range of (-21.00 to 5.00) mJ. The Delta H(sigma) measured was attributed a specific PEO-ion interaction associated with a polymer conformation change, both processes occurring at the interface phase. A significant effect of the electrolyte nature on the surface excess enthalpy was verified. Increasing the polymer size has an effect to release more energy for the ATPS interface formation process, while the substitution of PEO by L35, i.e, a hydrophobic increase on the top phase, makes more exothermic ATPS interface formation.