1. The thermodynamic isotope effect (TIE) may cause the distribution of isotopic molecules to differ substantially from the statistical distribution, especially in the case of light elements. 2. A method is indicated for setting up a system of algebraic equations, the solution of which makes it possible to calculate the concentration of isotopic molecules in any chemical compound consisting of any number of different isotopes. 3. A solution of the systems of algebraic equations is given for several simpler cases.