The essential contribution to the thermodynamics of open non-equilibrium systems was made by Ilya Prigogine, when he and his collaborators investigated systems of chemically reacting substances. The stationary states of such systems exists due to exchange of both particles and energy with the environment. In section 8 of the third chapter of his book, Prigogine has specified three contributions to the variation of entropy of the considered open system at the given volume and constant temperature formula_10. The increment of entropy formula_11 can be calculated according to the formula