The equations of concentration dependence of chemical or phase balance parameters are divided into two groups. The interpolation equations apply when properties of components or their limiting values are known. Such dependences are described by the Redlih-Kister equation. The extrapolation equations are used if limiting values of components or solutions properties were unknown beforehand. Such dependences can be described by means of Taylor’s sedate number. Six corresponding equations and examples of the mathematical description of experimental data are resulted.

Key words: liquid systems, mathematical model, given mistake.