Flowing post-discharges resulting from surfatron microwave discharges in nitrogen and methane gas mixture with low amount of methane (less than 1%) are implemented for nitrocarburising treatment. Atomic nitrogen and atomic carbon densities are measured in post-discharge by combining optical emission spectroscopy and NO titration. The kinetic schema leading to N and C atoms densities is presented with a special attention paid to the effect of gas temperature on the kinetic constants. The influence of different discharge parameters such as methane percentage, total pressure, methane injection upstream or downstream the discharge and gas temperature are studied by these techniques. It is shown that, except at the lowest percentage used in this study (0.02%), methane introduction decreases nitrogen atoms densities. A maximum in carbon atoms density (1013 cm−3) is found for 0.05% of methane introduced in a nitrogen discharge at 2670 Pa. The corresponding nitrogen atoms density is two orders of magnitude higher (2–3 × 1015 cm−3). We present some arguments showing that gas expansion is the only effect that gas temperature has on nitrogen atoms densities. At present, it is not clear if such explanation is also applicable to the effect of gas temperature on carbon atoms densities.