The applicability of the regular solution model for the description of the liquid-liquid equilibrium in ternary systems is discussed in the work and the conditions are established for the formation of two and three liquid phases. In the analysis it was found that certain forms of binodal curves cannot be obtained using only binary constants. On the other hand it appeared that the regular solution predicts a number of variants which have not been found experimentally yet. In the last part of the work, the effect of ternary constant is discussed as well.