Carbon nanotubes have been synthesized by catalytic chemical vapour deposition (CVD) of acetylene, over several iron-silica catalysts, in a fluidized bed reactor at 973 K. High yield and selective syntheses of carbon nanotubes was achieved with this reactor. The carbonaceous samples were submitted to acidic treatments (HF and/or HNO3), which clearly produced structural changes in the nanotubes structure, as observed by transmission electron microscopy (TEM). Moreover, the formation of oxidic groups on the surface of carbon nanotubes was followed by means of temperature desorption experiments and thermo-gravimetric analyses. Adsorption of hydrogen at 300 K and relative high pressures (up to 70 bar) has been also investigated on these purified carbon nanotubes, besides on different carbon materials. (C) 2007 Elsevier B.V. All rights reserved.