Liquid-liquid phase diagram of ionic liquid based 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim][BF4])+sodium dihydrogen phosphate/disodium hydrogen phosphate aqueous two-phase systems have been determined at 298.15 K, 303.15 K and 308.15 K. The phase diagrams and the liquid-liquid equilibrium (LLE) data of the [Emim][BF4]+sodium phosphate buffer aqueous two-phase systems were determined experimentally at different pH values at 298.15 K. The results show the stronger salt-out ability of Na2HPO4 than that of NaH2PO4, which could be related to the Gibbs free energy of hydration of the ions. The nonlinear equation developed by Merchuk was used to correlate the experimental binodal data. The tie-line data were correlated by Othmer-Tobias and Bancroft equation and a two-parameter equation. The results show a good agreement with the experimental data. The temperature dependence on the binodal curves and tie lines has been discussed. The effect of pH value of the sodium phosphate buffer on the phase diagram has also been studied. (C) 2012 Elsevier B.V. All rights reserved.