This paper presents a study of the role of hydrogen atoms (H) in the polymerisation of hydrocarbon gases occurring in the plasma volume and on the surface. These investigations are done in the newly designed double plasma experimental setup. It was found that an additional flux of H-atoms completely suppresses the nanoparticle formation in the plasma volume. It is shown that the creation of positive ions and neutrals follow closely the periodical formation of nanoparticles up to the moment when H-atoms are injected into the plasma. In the polymerisation process of hydrocarbon films H-atoms reduce the deposition rate (anti-synergistic effect) and enhance the etching rate (synergistic effect). The film properties are also influenced - the values of the optical constants (refraction index and extinction coefficient) decrease and the Raman spectra show almost complete destruction of the sp(2) graphitic structure due to the hydrogenation of the carbon network. (C) 2007 Elsevier B.V. All rights reserved.