Liquid-liquid equilibrium data for aqueous two-phase systems formed by a mixture of triblock copolymer L35 (M(n) = 1900 g.mol(-1)), sodium hydroxide or potassium hydroxide, and water were measured as a function of four temperatures, (283.15, 293.15, 303.15, and 313.15) K. The influences of the temperature and cation on the behavior of these systems were analyzed. Temperature variation affected more intensively the tie-line slope than the binodal position. The exchange of sodium cation for the potassium also leads to an increase of the two-phase region of the diagrams in each studied temperature. The reliability of all the tie-line experimental data was ascertained by applying the Othmer-Tobias correlation. The interaction parameters of the NRTL model and UNIFAC model group-interaction parameter were estimated for the liquid-liquid equilibrium at temperatures between (283.15 and 313.15) K, with a total of 80 tie-lines for each model. The results of the predicted liquid-liquid equilibrium data by NRTL and UNIFAC models showed that the NRTL was more able to represent the studied aqueous two-phase systems.