Results presented here concern the inlet CH 4 -CO 2 system used in our laboratory to deposit diamond films in microwave PACVD at 550-600 o C. Mass spectrometry associated to a molecular beam sampling is used to study both stable species (H 2 , CH 4 , C 2 H 2 , CO, CO 2 , ...) and radicals (H, OH, CH 3 , C 3 H 3 , C 3 H 5 , C 6 H 5 ...) produced in the plasma. The influence of the total gaseous flux is demonstrated here, for a given total pressure. It is shown that the residence time in the plasma is an important parameter, which controls the conversion degrees into hydrocarbon radicals, important etchant species (H and OH) and stable molecules. The general evolution observed for the main species can be explained by considering the conversion degrees of a few overall reactions. They allow better understanding of the formation of most minor species, either stable or radicals. The variation of the concentrations of some radicals (CH 3 , C 3 H 3 , C 6 H 5 ...) and of the first aromatic species like C 6 H 6 and C 7 H 8 , relatively to the concentration of H and OH is correlated to the deposition domain of carbon, the deposition kinetics and the variation of the diamond film structure and quality in terms of Csp 3 /(Csp 3 +Csp 2 ) ratio.