A diamond layer can be applied stably onto a graphite substrate in a CVD process when the graphite substrate is subjected to the following pretreatment steps before the CVD process: fine cleaning of the surface in a vacuum at a temperature >500° C., preferably >800° C., in an etching gas atmosphere, mechanical removal of loose particles, seeding of the substrate surface with very small diamond particles and at least one degassing treatment in a vacuum to remove adsorbed hydrocarbons and adsorbed air at a temperature T>500° C., preferably T>700° C.