The yield and nature of carbon nanostructures synthesized by the pyrolysis of acetylene using different quartz reactors and gas flow rates are investigated. The effect of these influential parameters on the growth of various carbon nanostructures has been studied and discussed. With a quartz reactor of inner diameter 24 mm and at gas flow rates of 40 and 600 seem of acetylene and argon. respectively, good quality carbon nanotubes with better yield have been obtained. The samples were characterized by X-ray diffraction, BET surface area measurements, scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. The hydrogenation behavior of these as-grown and purified carbon nanotubes was carried out at room temperature and in the pressure range 10-100 bar, using a high pressure Sieverts apparatus. The hydrogen storage capacity of carbon nanotubes was found to increase, after subjecting to acid treatment and heat treatment. The maximum hydrogen adsorption capacity of 2.1 wt% is obtained at 298 K and 100 bar for carbon nanotubes after acid treatment and air oxidation. (C) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.