The temperature dependence of the interaction of methyl radicals with the surface of a hard, amorphous hydrogenated carbon film is investigated using in situ real-time ellipsometry and infrared spectroscopy. This interaction is considered as an important process during plasma deposition of polymer-like hydrocarbon films or formation of polycrystalline diamond in methane-containing discharges. At room temperature CH3 adsorbs at sp(2)-coordinated CC bonds at the physical surface of the hard C:H film and forms a completely sp(3)-hybridized C:H adsorbate with a thickness of similar to0.17 nm. In the following, steady-state film growth is observed with a sticking coefficient of s(CH3)=10(-4). At a substrate temperature of T=570 K, incident CH3 causes net erosion with an etching yield of Y(CH3)=10(-4). At temperatures above 650 K the sticking coefficient of CH3 becomes positive again, leading to a graphite-like C:H adsorbate. CH3 adsorption is described by a reaction scheme based on the creation of dangling bonds at the film surface via abstraction of surface-bonded hydrogen by incoming CH3 radicals. These dangling bonds act as CH3 adsorption sites at room temperature or as a precursor for chemical erosion at elevated temperature.