The thermionic vacuum are (TVA) is investigated with emphasis on its application to the metallization of plastic surfaces. The TVA generates a pure, gas and macroparticle-free metal plasma. Due to its unique properties this discharge enables a simple control of important process parameters like the degree of ionization, deposition rate and ion energy. The ion energy, being of decisive importance for the adhesion of thin films, can be adjusted and controlled by parameters like discharge current and temperature of the heated cathode; the ion energy also depends on geometrical parameters like interelectrode distance and relative position of the electrodes. Thin copper films deposited on plastic substrates by means of the TVA show an excellent adhesion when the energy of the bombarding ions exceed 50 eV.