Phase diagram and liquid-liquid equilibrium (LLE) data for the 1-alkyl-3-methylimidazolium tetrafluoroborate [C(n)mim]BF4 (n = 2,3,4)) + (NH4)(2)SO4 H2O aqueous two-phase systems (ATPSs) were determined experimentally at T= (288.15, 298.15, and 313.15) K. The binodal curves were fitted by a four-parameter equation. On the basis of the empirical equation of the binodal curve with the highest accuracy and lever rule, the LLE data were calculated by MATLAB. The tie-lines were correlated by the Othmer-Tobias and Bancraft equations. In particular, the temperature dependence on the binodal curves and tie-lines was discussed, it was found that an decreasing in temperature caused the expansion of two-phase region. Furthermore, the effect of the ionic liquid structure on the phase equilibrium of ATPSs has been studied, the results showed that the ability of phase separation of the IL-based ATPSs follows the order [C(4)mim]BF4> [C(3)mim]BF4> [C(2)mim]BF4. (C) 2014 Elsevier B.V. All rights reserved.