Analysis of Mechanically Stimulated Gas Emission Spectrometry data combined with characterization of chemical composition, structure and mechanical properties of various hydrogenated amorphous carbon coatings has allowed shedding light on the possible reactions of tribochemical degradation. These reactions are conceivably driven by the mechanical deformation and stress rather than by frictional heating. A close correlation between the increase in proportion of sp3 CHx groups in the coating and the rate of CH4 emission was observed. Emission of methane is suggested to involve abstraction of methyl radical followed by recombination with hydrogen atom. Generation of ethane has to be due to recombination of two abstracted methyl radicals rather than an ethyl radical with hydrogen atom. The rate of H2 emission steeply and non-linearly increased with the hydrogen content in the coating. Hydrogen generation can involve both liberation of occluded H2 and re-combinative reaction.