A steel carburizing treatment using (Ar-CH4) gas mixtures with low CPI, contents is performed in an expanding microwave plasma reactor, where the distance between the discharge center and the substrate is choosen as a critical parameter to select the impinging plasma species. A 2 mu m thick zone oversaturated in carbon is identified by secondary ion mass spectometry measurements at the steel surface after a 15 min treatment in an (Ar-5%CH4) gas mixture. The amount of cal bon in this zone increases as the distance between the center of the discharge and the substrate decreases. After carburizing at a distance close to the center of the discharge, where CHx<3 and CxHy+ plasma species are numerous, almost no oxygen is detected. So, the plasma species improve the infusion of carbon partly by removing the oxides from the steel surface. Based on Fick's second law, a carbon diffusion coefficient of 10(-11) cm(2) s(-1) is found in the 2 mu m thick zone, whereas at a depth larger than 2 mu m. the classical carbon diffusion coefficient in austenite of 10(-7) cm(2) s(-1) is obtained. So, the storage of carbon in a thin zone close to the steel surface can be seen as an additional key stage in the plasma carburizing process.