Nanocrystalline dispersion systems on the Cu-base belong to the progressive materials that have particularly found their applications in the fields that place demands, besides specific physical and chemical properties of copper (electrical conductivity, thermal conductivity), on improved mechanical properties or on the resistance of mechanical properties against thermal degradation. The process of the formation of the secondary phase in the preparation process, as well as its stability after reduction of the CuO-MgO system to the Cu-MgO system are studied in the presented paper.