Amorphous hydrogenated carbon films (a-C:H) are usually deposited in low-temperature plasmas from a hydrocarbon precursor gas. The feed gas is dissociated and ionized in the plasma, and radicals and ions impinging onto the surface leading to film growth. Final stoichiometry and material properties depend strongly on composition, flux, and energy of the film forming species. Depending on process parameters, film properties range from polymer-like and soft films to hard and wear resistant coatings. Despite the great importance of this material for a wide range of applications, detailed knowledge on elementary mechanisms of film formation at the plasma-surface boundary is still lacking. One approach to isolate and to quantify individual growth mechanisms is to study selected surface processes in quantified radical-beam experiments. In recent years, such an experiment employing radical sources for atomic hydrogen (H) and methyl radicals (CH3) has been developed in our group. The interaction of these species with a-C:H films is monitored in real time by ellipsometry and infrared spectroscopy. The formation of polymer-like hydrocarbon films from beams of methyl radicals and atomic hydrogen is considered a model system for a-C:H growth in low-temperature plasmas from a hydrocarbon precursor gas. Ion-induced effects are not considered, because they are of minor importance for the formation of polymer-like C:H films. It is shown that the sticking coefficient of methyl radicals s(CH3) is only 10(-4) at room temperature and becomes negative 10(-4) at a substrate temperature of 600 K indicating film etching rather than film growth. Above 700 K, s(CH3) is again positive in the range of 10(-4) and the formation of a graphite-like C:H layer via CH, adsorption is observed. The low sticking coefficient of s(CH3) similar to 10(-4) can be enhanced by two orders of magnitude to s(CH3\H) similar to 10(-2), if an additional flux of atomic hydrogen is simultaneously interacting with the surface. This growth synergism is described by a model based on dangling bond creation via abstraction of surface bonded hydrogen by incoming atomic hydrogen, followed by chemisorption and incorporation of CH3. Methyl incorporation is a two-step process consisting of the formation of a tri-hydride terminated surface by CH3 chemisorption followed by hydrogen elimination due to hydrogen abstraction by incident H, which transforms tri- into mono-hydride surface groups. The existence of a partly tri-hydride terminated surface is directly observed via infrared spectroscopy. The process of forming or removing tri-hydrides can explain not only the measured growth rates and mutual interactions between CH3 and H in steady state, but also the dynamic of s(CH3\H) after a step-like change of the CH, and H fluxes. Based on a modeling of the experimental results using a set of rate equations, a cross-section between 2.4 and 5.4 Angstrom(2) for CH3 chemisorption at a dangling bond and a cross-section of similar to 10(-3) Angstrom(2) for abstraction of surface bonded hydrogen by CH3 are determined. The consequences of these results for the understanding of film formation in low-temperature plasmas are discussed.