The evolution of ternary mixtures containing water, poly(ethylene oxide) (PEO) and dextran or electrolytes (NaCl, Na2SO4 or Li2SO4), toward the formation of aqueous two-phase systems was investigated by calorimetry measuring enthalpies of solution for one of the components in water and in an aqueous solution of the other component. These values were then analyzed as enthalpies of transfer from water to aqueous solution of the second component, to probe the energy balance for the solvation of the transferred component between both states. The obtained results confirmed that enthalpic contributions are relevant to biphase formation in the presence of electrolytes, but much less important in mixtures of the two polymers. In all cases, phase separation was accompanied by an enthalpy increase, indicating that entropy increase is the driving force for aqueous two-phase systems formation. Moreover, based on enthalpy data, it was possible to assign different behavior to electrolytes capable of inducing aqueous two-phase systems formation (lithium and sodium sulfates), and sodium chloride, which does not induce phase separation. A model is proposed to account for the processes leading to phase separation in terms of electrolyte and polymer interaction.