Hydrogen is considered to be a clean energy carrier. However, the most serious barrier to potential uses is the development of feasible hydrogen storage systems. The discovery of high hydrogen storage capacity of carbon nanotubes (CNTs) makes up alternatives for hydrogen storage systems. In this study, the hydrogen adsorption on carbon nanotubes was investigated. CNTs were firstly synthesized by chemical vapor deposition (CVD) of acetylene (C(2)H(2)) on a magnesium oxide (MgO) powder impregnated with an iron nitrate (Fe(NO(3))(3)center dot 9H(2)O) solution. The synthesis parameters were selected as: the synthesis temperature of 800 degrees C, the iron content in the precursor of 5% and the synthesis time of 30 min. The liquid phase oxidation method was applied for the purification of the synthesized CNT materials. Three different chemicals-3 M HNO(3), H(2)SO(4), and HCl-were used in the removal of the metal catalysts from the synthesized CNT material to investigate the possible effects of each acid solution to the purification step. Purification of the synthesized CNT material was carried out under the conditions of 75 degrees C for 15 h. A 30% H(2)O(2):3 M HCl (1:1 v%) solution was also used in the purification step to remove both the metal catalysts and the amorphous carbon. The purification using this solution was performed at 75 degrees C for 8 h. The hydrogen storage capacities of the purified materials were measured using volumetric method. It was found that the hydrogen adsorption capacities of these materials were changed in the range of 0.60-4.86 wt% at the liquid nitrogen temperature and gas pressure up to 100 bar. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.