Diamond deposition in microwave CH4-CO2 plasmas depends on various parameters that change the nature and the concentration of the various species in the plasma. In this work emphasis was put on the effect exerted by changes in the power density injected in the plasma. These changes were obtained by varying both the microwave power and the pressure. The ro-vibrational temperature and then the kinetic temperature of the heavy species were determined by emission spectroscopy. Variations induced in the nature and the concentration of both stable species (H-2, CH4, CO, CO2,...) and radicals (H, OH, CH3, C3H3, C3H5,...) were derived from analyses performed with a mass spectrometer associated to a molecular beam sampling technique. Modeling based on various reactor configurations showed that the kinetics in the plasma is principally controlled by the gas temperature at the highest power density, whereas reactions with electrons are needed to initiate the mechanism at low power densities. The comparison of computed and experimental concentration profiles also shows that the influence of the residence time is of capital importance when CH4/CO2 plasmas are used for diamond deposition. This parameter allows explaining the correlation between the experimental conditions and the etching/deposition domains. (c) 2006 The Electrochemical Society.