The expression used in this article is to take into account the inverse transition of electrode’s material when it is selected in electro-spark processing; a formula is derived that estimates the ratio of solid-phase and liquid-vapor constituents of erosion. It is determined that as the selection criteria for the anode increase, the amount of transition of its material to the cathode in the liquid-vapor phase increases as well. When choosing the electrode material, the most important task to be taken into account is the solid-phase constituents of erosion.