With the aim of manufacturing 42CrMo4 steel by the electroslag remelting process (ESR) with a certain Ti and Al content and uniform distribution throughout the ingot, a study has been made of the complex metal-slag reactions which take place, especially of the most reactive elements. The thermodynamic activities of the elements in the molten metal and of the oxides in the slag were calculated and theoretic oxidation equilibria of the elements with greatest affinity for oxygen were proposed, as well as their exchange equilibria. Comparison of the theoretic equilibria with experimental results permits interpretation of how the system evolves during remelting and adjustment of the slag composition and other parameters in order to manufacture a steel of a certain composition and uniform from the bottom to the top of the ingot.