13C-NMR signals from carbon nanotubes (CNT) specially prepared using 13C enriched acetylene as reactant were analyzed. Carbon nanotubes (CNT) over several Fe/SiO2 catalysts were synthesized, by the decomposition of 13C2H2 using a pulsed method. The technique has been revealed as a powerful tool, not only to characterize the carbon species generated during CNTs synthesis. The quantification of carbide species in alternative admission of 13C labeled reactant allowed to obtain an interesting insight on the reaction mechanism of CNTs catalytically prepared.