Abstract: In a process for coating a plastic substrate 1 made of polymethylmethacrylate with metal using a direct current source 10 connected to an electrode disposed in an evacuable coating chamber 15, 15a and electrically connected to a target 3 to be sputtered, the adhesiveness and the service life of the layer are improved. In a first coating step, an argon plasma is maintained in the coating chamber 15, 15a for an extremely short period of time preferably until the so generated sputtering process passes from the oxidic to the metallic process. In a second coating step, helium is introduced in the coating chamber 15, 15a and a helium plasma is ignited. In a third coating step, argon is introduced into the coating chamber 15, 15a and an argon plasma is ignited and this argon plasma process is maintained until a desired thickness is reached.