Process and apparatus for pre-treatment of a substrate surface in a vacuum by a glow discharge for a subsequent coating process in a vacuum. The process includes maintaining a low pressure glow discharge between the substrate to be pre-treated and a counter-electrode, where the counter-electrode composed of at least a component of the coating to be deposited in the vacuum coating process. The process also includes periodically alternating a polarity of the substrate to act as a cathode or as an anode of the low pressure glow discharge, and individually controlling at least one of pulse length and discharge voltage in both polarities. A frequency of alternation of the polarity is set within a range of between 1 Hz and 1000 kHz. The apparatus includes an evacuatable vacuum chamber, a substrate holder positioned to hold a substrate to be pre-treated, at least one counter-electrode, and an alternating voltage generator coupled to the substrate to be pre-treated and the at least one counter-electrode. The substrate and the counter-electrode are mounted in a potential-free manner.