Carbon nanotube (CNT) growth mechanism was investigated by means of ion beam analysis. Multiwalled nanotubes (MWNTs) were formed by aerosol-assisted catalytic chemical vapor deposition (CCVD). This method is based on the pyrolysis of liquid aerosols containing hydrocarbons as carbon source and metallic precursor (ferrocene) as catalyst. For this study, a special procedure consisting of sequential injections of hydrocarbon containing or not 13 C isotope labels and ferrocene aerosol was developed in order to localize the carbon source corresponding to this sequence in the final carpet. The use of the nuclear reaction 13 C(d,p) 14 C induced by a microbeam with fast beam scanning allowed us to correlate at the micrometric scale the height in the CNT carpet with the chronology of the growth. Deuteron and proton ion induced X-ray emission were also performed in order to localize the iron element that originates from ferrocene. Results demonstrate unambiguously that nanotubes grow through a base-growth mechanism, where any new injection sequence leads to the growth of nanotubes directly on the substrate surface and that iron content is increasing along the MWNT carpet, with higher concentrations at both ends.