Liquid liquid equilibria of the aqueous two-phase systems of the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate and sodium dihydrogen phosphate/disodium hydrogen phosphate or their mixtures have been determined at 298.15 K, 303.15 K, and 308.15 K. The nonlinear equation developed by Merchuk was used to correlate the experimental binodal data. The two-phase regions gradually expand with the increase of the percentage of Na2HPO4 in the salt mixtures. It was also shown that the two-phase region decreases along with the temperature increases. The tie-line data were correlated by Othmer-Tobias and Bancraft equations. It was found that these equations fit the experimental data well. The temperature dependence on tie lines has been discussed. The effect of the concentration of the salt mixtures on binodal curves and tie-line data has also been discussed.