Abstract: In a process for applying a thin, transparent layer onto the surface of optical plastic elements for protecting the surface of such element against mechanical and chemical influences that surface is subjected to a monomeric vapor of organic compositions, preferably a silicon-organic substance in a vacuum container and a protection layer is separated from the vapor phase with the assistance of the radiation from an electrical gas discharge. Substances, in particular oxygen are added to the monomeric vapor during the polymerization which increases the layer hardness. The addition of these substances is performed with a delay with respect to the start of the polymerization process, so as to assume a good adherence of the layer on the surface.