Ionic liquids are becoming increasingly important as environmentally friendly solvents for extraction and reactions. To describe the equilibrium phenomenon of monomolecular solute + ionic liquid, a two-binary-interaction-parameter (TBIP) model is proposed based on excess Gibbs free energy derived from excess internal energy, which circumvents the difficulty of directly formulating excessive entropy. Different from conventional binary solutions, monomolecular solute + ionic liquid is a peculiar ternary solution, which theoretically needs six binary-interaction parameters. However, due to strong repulsive electrostatic forces between like-ions, the like-ions pairs are negligible in comparison with dislike-ion and molecule–ion pairs. When local electroneutrality is assumed, the necessary binary interaction parameters finally are reduced to only two. Tested against experimental data, our TBIP model shows a better precision for most solutions than a non-random two-liquid (NRTL) model. When correlated with only half points, the TBIP model has a better extrapolation performance, while the NRTL model fails to work for R1234ze(E)/[EMIM][BF4], ethanol/[MMIM][(CH3)2PO4], and water/[EMIM][Tf2N].