Abstract: Thermodynamic relationships are obtained to analyze patterns in the phase diagrams of systems with chemical interaction between materials in the vicinity of critical points. The procedure is based on introducing new variables (analogs of mole fractions). A general equation is derived for the isothermal–isobaric curve of chemical equilibrium in generalized concentration variables. The behavior of chemically reacting liquid–liquid systems in the vicinity of the critical points of two-phase equilibria is analyzed, and the corresponding diagrams are presented.