Analysis of scientific literature on the thermodynamics of the processes of oxidation of metals and reduction of their oxides indicates that results of thermodynamic calculations depend on the formulation of equations of chemical reactions. Chemical reactions are usually written per mole of oxygen or per mole of carbon dioxide, with the equilibrium constant and equilibrium composition of the gas phase calculated. The calculated values of standard Gibbs energies are used for reference only in this case. It was shown in our previous articles that calculation and comparison of standard Gibbs energies of various reactions is only possible if reactions are written per mole of atoms of the initial substances (per mole of the system). In this paper, the proposed method of calculation is shown using the example of reactions of reduction of iron oxides by carbon monoxide mixed with carbon dioxide at constant oxygen content in the system.