<p>Microarc surface alloying of steel is one of the most efficient methods of surface hardening of steel products. As the source of alloying elements, we use a&#xa0;coating pre-applied to the surface intended for hardened. Chemical compounds of alloying elements are, as a&#xa0;rule, used in the composition of the coating. To create coatings based on molybdenum carbide, it is advisable to analyze the possibility of using a&#xa0;low-cost and easily accessible substance, namely, ammonium molybdate, as a&#xa0;source of diffusant. As the most reliable method that can be used for this assessment, we can mention the thermodynamic analysis of possible chemical reactions. The aim of the present work is to determine, with the help of the thermodynamic analysis, the possibility and conditions of application of ammonium molybdate as a&#xa0;source of the diffusant in the process of microarc surface molybdenum plating of steel products.</p><p>We determined the variations of the Gibbs free energy and studied the temperature range of possible chemical reactions. The behavior of the Gibbs energy was carried out by the Temkin–Schwarzman method, which takes into account the temperature dependence of the heat capacities of substances participating in the reaction.</p><p>The results of the performed thermodynamic analysis of the chemical reactions of reduction of molybdenum trioxide under the conditions of microarc heating demonstrate that both its direct reduction and reduction with an intermediate stage of molybdenum-dioxide formation are possible. It was experimentally confirmed that a&#xa0;diffusion layer is formed by a&#xa0;solid solution of molybdenum and carbides. Under this layer, we observe a&#xa0;carbonized zone with pearlite structure followed by the original structure of 20&#xa0;steel. The maximum depth of the diffusion layer equal to 50–55 μm is observed for a&#xa0;volume concentration of the diffusant powder in the coating equal to 50% and its thickness of 1.0 mm.</p>